STKNX

Datasheet

Miniature KNX transceiver with voltage regulators and microcontroller support

Features
• Very thin fine pitch 4 x 4 mm VQFNPN24 package
• KNX certified, KNX TP1-256 supported
• Easy interface to microcontroller
• Very small system solution
• Two integrated voltage regulators for external use in application
– Selectable 3.3 V / 5 V - 20 mA linear regulator
VFQFPN24 (4 x 4 x 1.0 mm, 0.5 mm
pitch) – Adjustable 1 V to 12 V - 150 mA high- efficiency DC/DC step down
switching converter
• KNX bus power extractor supporting bus current up to 30 mA
• Adjustable KNX bus current slew rate dI/dt
• No crystal required
• Operating temperature range -40 °C to +85 °C

Applications
• KNX twisted pair network (KNX TP1-256)

Description
The STKNX is a transceiver device for KNX TP communication. The small package
and few external components enable very compact KNX node design.
Product status link The simple interface to the microcontroller allows easy replacement of physical layer
discrete component implementations.
STKNX
The STKNX device features two integrated voltage regulators for external use in the
Product summary application: the selectable 3.3 V / 5 V - 20 mA linear regulator and the adjustable 1 V
to 12 V - 150 mA high-efficiency DC/DC step down switching converter.
Order code STKNX STKNXTR
The integrated KNX bus power extractor supports bus current up to 30 mA to power
Package VFQFPN24
external devices and the STKNX transceiver's own power needs, while limiting the
Packing Tube Tape & Reel bus current slew rate according to KNX specifications.
The STKNX ensures safe coupling to the bus and provides the bus monitoring
Product label
warning for the loss of bus power.

DS12399 - Rev 3 - May 2021 www.st.com

For further information contact your local STMicroelectronics sales office.
 STKNX
Typical application circuit and block diagram

1 Typical application circuit and block diagram

1.1 Typical application circuit

Figure 1. Typical application circuit, buck converter enabled, linear regulator supplied by impedance
modulator

DCDC DCDC DCDC DCDC

FB GND LX IN
DCDC_SS VDDHV_PD

VDDHV
VREF

VDD_REGIN VFLT

KNX_AC
VCCCORE

STKNX VGATE

KNX_A
VCC_OK

R68
KNX_OK

CPH
KNX_TX

KNX_RX CPL

VCC_SEL KNX_B KNX_B

Note: VOUT range 1 V - 12 V.

VCORE selectable to 3.3 V / 5 V through VCC_SEL (3.3 V in the example).

DS12399 - Rev 3 page 2/27

 STKNX
Typical application circuit

Figure 2. Typical application circuit, buck converter enabled, linear regulator supplied by buck converter

DCDC DCDC DCDC DCDC

FB GND LX IN
DCDC_SS VDDHV_PD

VDDHV
VREF

VDD_REGIN VFLT

KNX_AC
VCCCORE

STKNX VGATE

KNX_A
VCC_OK

KNX_OK R68

KNX_TX CPH

KNX_RX CPL

VCC_SEL KNX_B KNX_B

Note: The VOUT level needs to be compliant with VDD_REGIN recommended operating conditions.
VCORE selectable to 3.3 V / 5 V through VCC_SEL (3.3 V in the example).

DS12399 - Rev 3 page 3/27

 STKNX
Typical application circuit

Figure 3. Typical application circuit, buck converter disabled

DCDC DCDC DCDC DCDC

FB GND LX IN
DCDC_SS VDDHV_PD

VDDHV
VREF

VDD_REGIN VFLT

KNX_AC
VCCCORE

STKNX VGATE

KNX_A
VCC_OK

KNX_OK R68

KNX_TX CPH

KNX_RX CPL

VCC_SEL KNX_B KNX_B

Note: VCORE selectable to 3.3 V / 5 V through VCC_SEL (3.3 V in the example).

DS12399 - Rev 3 page 4/27

 STKNX
Typical application circuit

Figure 4. Typical application circuit, linear regulator disabled

DCDC DCDC DCDC DCDC

FB GND LX IN
DCDC_SS VDDHV_PD

VDDHV
VREF

VDD_REGIN VFLT

KNX_AC
VCCCORE

STKNX VGATE

KNX_A
VCC_OK

KNX_OK R68

KNX_TX CPH

KNX_RX CPL

VCC_SEL KNX_B KNX_B

Note: The VOUT level needs to be compliant with VCCCORE recommended operating conditions.
VCC_SEL needs to be set according to the VCCCORE level (3.3 V in the example).

Table 1. External components typical value

Reference Type Typ. value Rating Description

Capacitors

CPH MLCC 100 nF VRATED ≥ 50 V Equalizer storage capacitor

CPL MLCC 100 nF VRATED ≥ 50 V Equalizer storage capacitor

MLCC or Impedance modulator storage capacitor

CGATE 10 µF - 47 µF VRATED ≥ 10 V
electrolytic - see Table 7

CAC MLCC 10 nF VRATED ≥ 50 V Bus AC coupling capacitor

Impedance modulator output bulk

CVDDHV Electrolytic ≥ 100 µF VRATED ≥ 35 V capacitor
- see Table 7
Impedance modulator compensation
CFH MLCC 47 nF VRATED ≥ 35 V
capacitor

DS12399 - Rev 3 page 5/27

 STKNX
Typical application circuit

Reference Type Typ. value Rating Description

Impedance modulator compensation

CFL MLCC 47 nF VRATED ≥ 35 V
capacitor
Buck converter input decoupling
CIN MLCC 10 µF VRATED ≥ 35 V
capacitor

COUT MLCC 22 µF VRATED > VOUT Buck converter output capacitor

Buck converter soft-start time

CSS MLCC 10 nF - 470 nF VRATED ≥ 6.3 V
programming capacitor - see Eq. (3)

CREF MLCC 470 nF VRATED ≥ 35 V VREF decoupling capacitor

CCORE MLCC 4.7 µF VRATED ≥ 6.3 V Linear regulator output capacitor

1/(2π * RFB1 * 28 kHz)

CFB1 MLCC VRATED ≥ 16 V Buck converter compensation capacitor
NM if RFB1 = 0 Ω
Resistors
RDIS Resistor 4.7 MΩ - Reverse polarity discharging resistor
RSH Resistor 1 kΩ - Series resistor to CPH
RSL Resistor 1 kΩ - Series resistor to CPL

RTX Resistor 68 Ω PDISS ≥ 1 W Tx current limiting resistor

(VOUT / 1 V - 1) * RFB2 Buck converter output voltage adjusting

RFB1 Resistor -
0 Ω for VOUT = 1 V resistor

10 kΩ typ. (< 100 kΩ) Buck converter output voltage adjusting

RFB2 Resistor -
NM for VOUT = 1 V resistor

Inductors
IR > 150 mA
L Power inductor 33 µH Buck converter output inductor
ISAT > 550 mA

Diodes
VBR > 50 V Input diode (protection from reverse
D1 Diode LL4148 or equivalent
VF (50 mA) < 1 V polarity connection)

SMAJ40CA or lower clamping

D2 TVS - Transient voltage suppressor diode
voltage
VBR > 50 V
D3 Diode LL4148 or equivalent Output diode
VF (50 mA) < 1 V

VBR > 50 V Optional KNX_AC clamping diode.

D4 Diode LL4148 or equivalent Recommended for noise immunity
VF (50 mA) < 1 V improvement in noisy environment.

DS12399 - Rev 3 page 6/27

 STKNX
Block diagram

1.2 Block diagram

Figure 5. STKNX Block diagram

DS12399 - Rev 3 page 7/27

 STKNX
Pin connection and pin description

2 Pin connection and pin description

2.1 Pin connection

Figure 6. Pin connection (top view)

2.2 Pin description

Table 2. STKNX Pin description

Pin Pin name Function

Linear regulator supply input. Short to VCCCORE to disable the linear regulator and supply VCCCORE
1 VDD_REGIN
externally.
2 KNX_AC BUS AC-coupled sense for the Rx input and Tx feedback. DC biased to 9.7 V typ.
3 R68 KNX transmitter output
4 KNX_B Analog ground
5 CP_H Equalization cap connection to KNX supply (KNX+)
6 CP_L Equalization cap connection to KNX ground (KNX-)
7 VFLT Impedance modulator compensation
8 VREF Impedance modulator reference
9 VDDHV VDDHV supply input and impedance modulator feedback
10 VDDHV_PD Impedance modulator power output
11 KNX_A KNX power supply input (KNX+)
12 KNX_B Analog ground
13 VGATE Impedance modulator storage capacitor connection

DS12399 - Rev 3 page 8/27

 STKNX
Pin description

Pin Pin name Function

14 DCDC_IN Step down converter supply input. Short to ground or leave floating to disable the switching converter.
15 DCDC_LX Step down converter switching output
16 DCDC_GND Step down converter power ground
17 DCDC_SS Step down converter soft-start programming pin
18 DCDC_FB Step down converter feedback input. Sets output voltage (1 V - 12 V range) through the resistor divider.
19 VCC_OK VCCCORE power good CMOS digital output
20 VCCCORE Linear regulator output (3.3 V / 5 V selectable). Supply voltage for digital I/O.
21 KNX_OK KNX bus power good CMOS digital output
Selects linear regulator output voltage. Internally pulled down (6 µA typ.).
22 VCC_SEL
Tie to VCCCORE to select 5 V. Short to ground or leave floating to select 3.3 V.
23 KNX_RX Receiver CMOS digital output
24 KNX_TX Transmitter digital input. Internally pulled down (6 µA typ.).
Connect to analog ground.
- Exposed pad For thermal optimization, maximize the area of the ground layer on which the exposed pad is soldered
and provide good thermal connection with the bottom ground layer through vias.

DS12399 - Rev 3 page 9/27

 STKNX
Thermal characteristics

3 Thermal characteristics

Table 3. Thermal characteristics

Symbol Parameter Test condition Value Unit

TJ Maximum operating junction temperature - 110 °C

TAMB Operating ambient temperature - -40 to 85 °C

TSTG Storage temperature - -50 to 150 °C

Mounted on a 2s2p PCB, with a dissipating surface

Thermal resistance junction to ambient,
RthJA connected through vias on the bottom side of the 35 °C/W
steady state
PCB.

DS12399 - Rev 3 page 10/27

 STKNX
Electrical specifications

4 Electrical specifications

4.1 Absolute maximum ratings

Table 4. Absolute maximum ratings

Symbol Parameter Min. Max. Unit

KNX_A KNX supply input -0.3 45 V

VDD_REGIN Linear regulator supply input -0.3 40 V
VDDHV supply input and impedance modulator
VDDHV -0.3 40 V
feedback
VREF Impedance modulator reference -0.3 40 V
KNX_B, DCDC_GND Variation between different ground pins -0.3 0.3 V
CP_H Equalizing cap. high -0.3 KNX_A + 0.3 V
CP_L Equalizing cap. low -0.3 KNX_A + 0.3 V
R68 KNX transmitter output -0.3 KNX_A + 0.3 V
KNX_AC BUS AC-coupled sense -0.3 KNX_A + 0.3 V
VDDHV_PD Impedance modulator power output -0.3 KNX_A + 0.3 V
Max. (-0.3,
VGATE Impedance modulator coupling cap. KNX_A + 0.3 V
KNX_A - 7.2)
VCCCORE Linear reg. output. I/O supply. -0.3 5.5 V
KNX_TX Transmitter digital input -0.3 Min. (5.5, VCCCORE + 0.3) V
KNX_RX Transmitter digital output -0.3 Min. (5.5, VCCCORE + 0.3) V
KNX_OK KNX bus power good -0.3 Min. (5.5, VCCCORE + 0.3) V
VCC_SEL 3.3 V / 5 V selection for linear reg. -0.3 Min. (5.5, VCCCORE + 0.3) V
VCC_OK VCCCORE power good -0.3 Min. (5.5, VCCCORE + 0.3) V
VFLT Impedance modulator compensation -0.3 5.5 V
DCDC_IN Step down converter input -0.3 40 V
DCDC_LX Step down converter switching node -0.3 Min. (40, DCDC_IN + 0.3) V
DCDC_FB Step down converter feedback -0.3 3.6 V
DCDC_SS Step down converter soft-start programming pin -0.3 3.6 V

4.2 Recommended operating conditions

Table 5. Recommended operating conditions

Symbol Parameter Test condition Min. Max. Unit

KNX_A KNX supply input(1) - 20 32 V

VCCCORE I/O supply and linear reg. output - 3 5.5 V

DCDC_IN Step down converter input - 13 32 V

VDD_REGIN Linear regulator supply input(2) VCC_SEL shorted to GND 6.8 32 V

DS12399 - Rev 3 page 11/27

 STKNX
Electrical characteristics

Symbol Parameter Test condition Min. Max. Unit

VDD_REGIN Linear regulator supply input(2) VCC_SEL shorted to

8.5 32 V
VCCCORE
IVDDHV_PD Continuous output current from VDDHV pin(3) - - 30 mA

IREG Continuous output current from VCCCORE pin(3) - - 20 mA

IDCDC Continuous output current from DCDC switching converter (3) - - 150 mA

1. Indicates DC value. With the active and equalization pulse bus voltage must be between 11 V and 45 V.
2. Short VDD_REGIN to VCCCORE to disable the internal linear regulator and provide VCCCORE voltage externally.
3. The maximum current capability refers to the voltage regulator only. The usable current capability can be limited by the KNX
bus current consumption specification.

4.3 Electrical characteristics

Table 6. Electrical characteristics

Parameters given for a device operating within the recommended operating conditions, unless otherwise specified. Typical
values are referred to TJ = 27 °C.
Symbol Parameter Conditions Min. Typ. Max. Unit

Power supply
Excluding active and equalization
V(KNX_A) DC supply voltage on KNX_A pin 20 - 32 V
pulse
V(KNX_A) = 32 V, no activity on
Bus current consumption, no
bus, no transmission, no external - 1 - mA
load
load
I(KNX_A) V(KNX_A) = 32 V, no activity on
Bus current consumption, 30 mA bus, no transmission, 30 mA load
- 31 - mA
load on VDDHV rail (including linear
regulator and switching converter)
KNX_OKRIS KNX_OK rising threshold
VREF rising - - 13.5 V
(VREF) referred to VREF voltage

KNX_OKFALL KNX_OK falling threshold

VREF falling 9.7 - - V
(VREF) referred to VREF voltage

KNX_OKHYST KNX_OK hysteresis referred to

- - 1.4 - V
(VREF) VREF voltage

KNX_OKRIS KNX_OK rising threshold KNX_A rising slowly, VREF

- - 18.5 V
(KNX_A) referred to KNX_A DC voltage settled

Impedance modulator
V(KNX_A) = 20 V DC
V(KNX_A) - V(VDDHV)
VDDHV drop ILOAD = 5 mA - - 6.5 V
voltage drop
D3 = LL4148 or equivalent
V(KNX_A) = 20 V DC,
V(KNX_A) - V(VDDHV)
VDDHV drop ILOAD = 30 mA, - - 7.2 V
voltage drop
D3 = LL4148 or equivalent
V(KNX_A) - V(REF) V(KNX_A) = 20 V DC, VREF
VREF drop 3 4.3 5 V
voltage drop settled

VCCCORE voltage and linear regulator

VCC_SEL shorted to GND 3 3.3 3.6 V
VCCCORE Regulated voltage
VCC_SEL shorted to VCCCORE 4.5 5 5.5 V

VCC_OKRIS VCC_OK rising threshold VCC_SEL shorted to GND 2.3 - 2.8 V

DS12399 - Rev 3 page 12/27

 STKNX
Electrical characteristics

Symbol Parameter Conditions Min. Typ. Max. Unit

VCC_OKRIS VCC_OK rising threshold VCC_SEL shorted to VCCCORE 3.3 - 4 V
VCC_SEL shorted to GND 2.0 - 2.5 V
VCC_OKFALL VCC_OK falling threshold
VCC_SEL shorted to VCCCORE 2.9 - 3.6 V
VCC_SEL shorted to GND - 0.3 - V
VCC_OKHYST VCC_OK hysteresis
VCC_SEL shorted to VCCCORE - 0.4 - V
VCCSEL internal pull- down
IVCCSEL - - 6 - µA
current
Programmable DC-DC switching converter
VIN Input voltage at DCDC_IN pin - 13 - 32 V

VOUT Output voltage adjusting range - 1 - 12 V

VFB Feedback voltage reference - 0.9 1 1.1 V

Undervoltage lockout rising

UVLORIS VIN rising 9 10 11 V
threshold on VIN voltage
Undervoltage lockout falling
UVLOFALL VIN falling 5.4 6 6.6 V
threshold on VIN voltage
Undervoltage lockout hysteresis
UVLOHYST - - 4 - V
on VIN voltage
ILIM High side MOSFET current limit - 325 650 975 mA

13 V < VIN < 32 V

3.3 V < VOUT < 12 V
VOUT_RIP Output voltage ripple - 50 - mV
IOUT = 5 mA ~ 150 mA
COUT = 22 µF MLCC(1)

High side MOSFET on

- - 1.9 -
RDS(ON) resistance Ω
Low side MOSFET on resistance - - 1.2 -

OTP Overtemperature protection Junction temperature(2) 110 140 - °C

Overvoltage protection rising

OVPRIS FB voltage rising 1.1 1.25 1.4 V
threshold on FB
Overvoltage protection falling
OVPFALL FB voltage falling 0.95 1.1 1.25 V
threshold on FB
ISS Current sourced from SS pin During soft-start - 2.5 - µA

Transmitter
RDS(ON) Tx MOSFET on resistance - - 5 - Ω

KNX_TX internal pull- down

IKNX_TX - - 6 - µA
current
Digital I/Os

Maximum voltage level that will VCCCORE = 3.3 V 0.7 - -

VIL V
be interpreted as a logic 0 VCCCORE = 5 V 1.2 - -

Minimum voltagelevel that will be VCCCORE = 3.3 V - - 2.2

VIH V
interpreted as a logic 1 VCCCORE = 5 V - - 3
VCCCORE = 3.3 V
0 - 0.4 V
ISUNK = 300 µA
VOL Logic low output level
VCCCORE = 5 V
0 - 0.4 V
ISUNK = 400 µA

DS12399 - Rev 3 page 13/27

 STKNX
Electrical characteristics

Symbol Parameter Conditions Min. Typ. Max. Unit

VCCCORE = 3.3 V, VCCCORE

- VCCCORE V
ISOURCED = 300 µA - 0.5
VOH Logic high output level
VCCCORE = 5 V, VCCCORE
- VCCCORE V
ISOURCED = 400 µA - 0.5

1. Not tested in production. Guaranteed by design.

2. Not tested in production. Based on characterization.

DS12399 - Rev 3 page 14/27

 STKNX
Device description

5 Device description

The STKNX is a transceiver device for twisted pair communication, following the KNX twisted pair standard (KNX
TP1-256). Detailed information on the KNX bus can be found in the KNX standards and on the KNX website
(www.knx.org).
The STKNX is composed of two main functional blocks: the bus interface and the voltage regulators.
• The bus interface consists of the transmitter, receiver and impedance modulator
• The voltage regulators block consists of an adjustable output voltage step down switching converter with
integrated power MOSFETs and a 3.3 V / 5 V programmable linear regulator

Figure 7. KNX bus voltage and corresponding digital signals

VBUS

Veq

DC level
Vact

Active pulse Equalization pulse t

Logic 0 Logic 1

35 69
104 104

KNX_RX

KNX_TX
(if transmitting)

5.1 Bus interface

The bus interface connects the STKNX to the KNX bus for transmitting, receiving and extracting power.
Through the bus interface, the STKNX supports
• Interfacing a microcontroller with the KNX bus, translating signals between the logic level domain and KNX
bus domain
• Extracting power from the bus to supply the STKNX itself, the microcontroller and application devices
The KNX standard specifies a bit period of 104 µs. It defines the logic 1 as the idle state of the bus (DC voltage
level between 21 V and 32 V), the logic 0 (also called active pulse) as a voltage drop of the bus.
The active pulse is generated by the transmitter. Ideally, the drop has a rectangular shape, a depth between 6 V
and 9 V and a duration of 35 µs. Each active pulse is followed by an equalization phase of 69 µs typical duration,
which consists in an overshoot of the bus voltage above the DC level, followed by an exponential decay.
See the KNX Twisted Pair Standard (KNX TP1-256) for more detailed information.

DS12399 - Rev 3 page 15/27

 STKNX
Voltage regulators

5.1.1 Transmitter
The transmitter converts logic level signals received at the KNX_TX pin to analog signals on the KNX bus. To
transmit a logic 1 (equivalent to transmitter in idle state), the KNX_TX pin has to be kept low for 104 µs. To
transmit a logic 0, the KNX_TX has to be forced high for 35 µs (active pulse) and then low for 69 µs.
During the active pulse, the transmitter forces a voltage drop of 7.5 V typ. on the KNX bus, by sinking current
through the R68 pin.

5.1.2 Receiver
The receiver detects the beginning and the end of the active pulse and provides a logic level output on the
KNX_RX pin. The KNX_RX pin is high during the active pulse, low during the equalization phase and idle state.
The detection threshold for the start of the active pulse is 0.6 V typ. below the bus DC voltage.

5.1.3 Impedance modulator

The KNX standard allows a bus voltage ranging from 21 V to 32 V (DC component). The bus provides supply
for the STKNX and is the communication medium. During transmission, a -10.5 V / +13 V AC component can be
superimposed to the DC component mentioned above.
Moreover, the KNX standard specifies that each module connected to the bus has to show a controlled
impedance and to limit the bus load current slope dI/dt, while not transmitting.
The impedance modulator purpose is to extract power from the KNX bus in order to supply STKNX integrated
voltage converters and the application on the KNX module, while ensuring compliance to KNX impedance
specifications.
In particular, impedance modulator:
• Extracts a stable power rail (VDDHV) from the KNX bus DC level
• Smooths any load change applied at its output, limiting dI/dt on the bus current
• Controls the impedance of the bus device during the active pulse and the equalization pulse according to
KNX standard requirements
Since the current drawn from the bus must change very slowly, abrupt load current steps from the load applied to
the STKNX have to be absorbed by the large filter capacitor on VDDHV rail (CVDDHV on Figure 1), which should
be sized accordingly.
The bus current slope limit is controlled through CGATE (Figure 1).
CGATE = 47 µF sets a slope lower than 0.5 mA/ms, compliant to the KNX requirement for fan in model up to the
10 mA bus load.
For a higher fan in, it can be useful to set a higher current slope limit, in order to manage wider load changes
minimizing the CVDDHV value. That can be done by reducing the CGATE value proportionally to the desired
slope limit increase.
Table 7 shows recommended CGATE and CVDDHV values for the minimum and maximum fan in.

Table 7. Recommended CGATE and CVDDHV vs. fan-in

Fan in Recommended CGATE Recommended minimum CVDDHV

10 mA 47 µF 100 µF
30 mA 10 µF - 47 µF 220 µF

5.2 Voltage regulators

The STKNX features two integrated voltage regulators for external use in the application:
• a linear regulator with 3.3 V or 5 V selectable output voltage, 20 mA current capability
• a step down switching converter with 1 V - 12 V adjustable output voltage, 150 mA current capability.

5.2.1 3.3 V / 5 V linear regulator

The linear regulator converts the input voltage on the VDD_REGIN pin to 3.3 V or 5 V output on the VCCCORE
pin. The output voltage level is selectable by the VCCSEL pin.

DS12399 - Rev 3 page 16/27

 STKNX
Voltage regulators

• VCCSEL tied to GND → VCCCORE = 3.3 V

• VCCSEL tied to VCCCORE → VCCCORE = 5 V
VCCSEL should not be changed when STKNX is operational.
The output current capability is 20 mA. A 4.7 µF capacitor or higher is required between VCCCORE and KNX_B
for stability.
VCCCORE is also the supply input for STKNX digital I/Os. The linear regulator can be disabled by shorting
VCCCORE to VDD_REGIN; in that case VCCCORE voltage to supply I/Os has to be provided externally and
VCCSEL has to be configured according to the voltage level (3.3 V or 5 V).

5.2.2 Buck converter

The STKNX integrates a high-efficiency low-consumption buck switching converter.
The switching converter is supplied from the DCDC_IN pin, connected to VDDHV rail in the typical application.
When voltage at the DCDC_IN pin is lower than UVLO, the switching converter is disabled.
Buck converter output voltage is adjustable between 1 V and 12 V by means of an external resistor divider on the
DCDC_FB pin, according to the following expression:
Vout = 1V ⋅ 1 + RFB1/RFB2 (1)
Where RFB1 and RFB2 are the upper and lower resistor of the divider respectively (see Figure 1). The RFB2
typical value is 10 kΩ (RFB2 values higher than 100 kΩ should be avoided).
To set VOUT = 1 V, RFB1 should be 0 Ω and RFB2 not mounted.
In the usual case of the low ESR ceramic capacitor as the output capacitor for the converter, it is recommended to
add an external feedforward compensation capacitor CFB1 in parallel to RFB1, for VOUT > 1 V.
The CFB1 default value can be calculated according to the following expression:
1
CFB1 = (2)
2π ⋅ RFB1 ⋅ 28kHz
The buck converter can deliver a continuous output current up to 150 mA, however the maximum current
capability will not always be usable. In fact, at the application level, the KNX bus current consumption must stay
within the KNX specification.
The buck converter implements soft-start to prevent a high inrush current at start-up. Soft-start time TSS is
programmable through the external capacitor CSS between the DCDC_SS pin and GND, according to the
following expression:
C
TSS = 1V ⋅ SS (3)
ISS

where ISS is 2.5 µA typ.

The buck converter features a full set of protections, which includes overtemperature protection (OTP),
overcurrent protection (OCP) and overvoltage protection (OVP).

DS12399 - Rev 3 page 17/27

 STKNX
Layout recommendations

6 Layout recommendations

PCB layout is an important part of DC-DC switching converter design. A poor board layout can compromise
important parameters of the DC-DC converter such as efficiency, output voltage ripple, line and load regulation
and stability.
Good layout for the STKNX can be implemented by following the few simple design rules listed in this section.
These rules have been applied to the STKNX routed area on the STKNX evaluation board (EVALKITSTKNX),
where only the TOP and BOTTOM layers have been used from the four available, so it can be transposed
on a low cost 2-layer PCB. It is then easy to implement the rules on a final KNX product. The source files of
EVALKITSTKNX PCB layout are available for download from www.st.com.

Figure 8. STKNX area routed using top and bottom layers only

Refer to Figure 9 and Figure 10 below for the recommendations described below:
• Place CIN (C13) close to the STKNX and connect it between pins VIN and DCDC_GND directly on top layer
(DCDC_LX trace crosses between CIN pads)
• Connect COUT (C24) to DCDC_GND directly on top layer
• Keep the following power loops short:
– CIN → DCDC_IN → DCDC_LX → L1 → COUT → CIN (green)
– COUT → DCDC_GND → DCDC_LX → L1 → COUT (red)
– CIN → DCDC_IN → DCDC_GND → CIN (purple)
• Use properly sized traces or shapes for power paths (DCDC_IN, DCDC_GND, VDCDC, LX)
– Keep FB/Feedback and SS/Soft-Start components (Rfbx, Cfb1, Css) away from switching / noisy node
(DCDC_LX), shielding with quiet nets (DCDC_GND in the image) is recommended (black)
– Connect DCDC_GND pin (16) and KNX_B pins (4 and 12) to the exposed pad shape below the IC, as
shown in Figure 10, to ensure ground consistency
– Place several GND vias on STKNX package exposed pad (x9 in EVALKITSTKNX).

DS12399 - Rev 3 page 18/27

 STKNX
Layout recommendations

Figure 9. Layout recommendations description

Figure 10. Layout recommendations application

DS12399 - Rev 3 page 19/27

 STKNX
Package information

7 Package information

In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK packages,
depending on their level of environmental compliance. ECOPACK specifications, grade definitions and product
status are available at: www.st.com. ECOPACK is an ST trademark.

7.1 VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package information

Figure 11. VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package outline

DS12399 - Rev 3 page 20/27

 STKNX
VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package information

Table 8. VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package mechanical data

Dimensions [mm]
Symbol
Min. Nom. Max.

A 0.80 - 1.00
A1 0.00 - 0.05
A2 - 0.65 -
A3 - 0.20 -
b 0.20 0.25 0.30
D 3.9 4.0 4.1
D2 2.7 2.8 2.9
e - 0.5 -
E 3.9 4.0 4.1
E2 2.7 2.8 2.9
L 0.30 0.35 0.40
k 0.20 - -
N - 24 -
Symbol Tolerance of form and position [mm]
aaa 0.15
bbb 0.10
ccc 0.10
ddd 0.05
eee 0.08
fff 0.10

DS12399 - Rev 3 page 21/27

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VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package information

Figure 12. Suggested footprint

DS12399 - Rev 3 page 22/27

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Revision history

Table 9. Document revision history

Date Version Changes

08-Feb-2018 1 Initial release.

Throughout document:
- layout and template changes
06-Dec-2020 2
- minor text edits
Added Section 6 Layout recommendations
20-May-2021 3 Change to Equation 1 in Section 5.2.2

DS12399 - Rev 3 page 23/27

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Contents

Contents
1 Typical application circuit and block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Pin connection and pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Thermal characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

4 Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.1 Absolute maximum ratings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3 Electrical characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Device description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

5.1 Bus interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1.1 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.1.2 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.1.3 Impedance modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2 Voltage regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2.1 3.3 V / 5 V linear regulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2.2 Buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

6 Layout recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

7 Package information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
7.1 VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

DS12399 - Rev 3 page 24/27

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List of figures

List of figures
Figure 1. Typical application circuit, buck converter enabled, linear regulator supplied by impedance modulator . . . . . . . . 2
Figure 2. Typical application circuit, buck converter enabled, linear regulator supplied by buck converter . . . . . . . . . . . . . 3
Figure 3. Typical application circuit, buck converter disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 4. Typical application circuit, linear regulator disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 5. STKNX Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 6. Pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 7. KNX bus voltage and corresponding digital signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 8. STKNX area routed using top and bottom layers only . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 9. Layout recommendations description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 10. Layout recommendations application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 11. VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 12. Suggested footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

DS12399 - Rev 3 page 25/27

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List of tables

List of tables
Table 1. External components typical value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Table 2. STKNX Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 3. Thermal characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 4. Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 5. Recommended operating conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 6. Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 7. Recommended CGATE and CVDDHV vs. fan-in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 8. VFQFPN 4 x 4 x 1.0 24 pitch 0.5 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Table 9. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

DS12399 - Rev 3 page 26/27

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DS12399 - Rev 3 page 27/27