End of Life.

Last Available Purchase Date is 31-Dec-2014

Si9121
Vishay Siliconix

High-Voltage, Non-Isolated Buck-Boost Converter

for ISDN Digital Phones

FEATURES
Fixed +5-V or +3.3-V Output −10-V to −60-V Input Voltage Range Current Mode Control
Integrated Floating Feedback Amplifier 95-kHz PWM Operation Hiccup Mode Short Circuit
On-Chip 70-V, 1.5- N-Channel Integrated Soft-Start and Oscillator Protection
MOSFET Switch High Efficiency Over Full Load Range Thermal Shutdown
Integrated High Voltage Start-Up Circuit, Under Voltage Lockout SOIC-8 Narrow-Body Package
with VCC Regulator

DESCRIPTION
The Si9121 simplifies the −48-V to +5-V or +3.3-V converter In order to reduce external component count, the Si9121 has
design for ISDN application by integrating the floating a fully integrated 95-kHz oscillator and soft-start circuit.
feedback error amplifier providing direct output voltage
regulation. This approach eliminates the need for an external
shunt regulator. The Si9121 also integrates a high voltage The Si9121 is available in both standard and lead (Pb)-free
depletion mode MOSFET which allows the converter to be SOIC-8 pin packages, and is offered in either +5-V or +3.3-V
powered directly from the high input bus voltage without fixed output options (Si9121DY-5 or Si9121DY-3,
requiring an external start-up circuit. Combined with simple respectively). In order to satisfy the stringent ambient
magnetic design due to its non-isolated topology, the Si9121 temperature requirements in many applications, the Si9121 is
provides a one-chip solution for complete ISDN power supply. rated to the industrial temperature range of −40 C to 85 C.

FUNCTIONAL BLOCK DIAGRAM

GND

VCC Regulator
VCC

Reference Generator

VOUT
BYPASS
VOUT
LX +5 V/400 mA
or
+3.3 V/400 mA
Control

COMP VNEG CS

VNEG
−48 V

Document Number: 71112 www.vishay.com

S-40708—Rev. C, 19-Apr-04 1
Si9121
Vishay Siliconix

ABSOLUTE MAXIMUM RATINGS (ALL VOLTAGES REFERENCED TO GND = 0 V)

VNEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . −63 V Operating Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150_C
VCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG −0.3 V to VCC + 0.3 V Power Dissipation (Package)a
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG + 13.2 V 8-Pin SOIC (Y Suffix)b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.25 W
ILX (peak current ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 A
Thermal Impedance (QJA)a
VOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V
8-Pin SOIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 _C/W
Bypass, CS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG −0.3 V to VCC +0.3 V
Notes
(VLX − VCS ) internal power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 V a. Device mounted with all leads soldered or welded to PC board.
Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . −65 to 150_C b. Derate 10 mW/_C above 25_C.

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation
of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

RECOMMENDED OPERATING RANGE (ALL VOLTAGES REFERENCED TO GND = 0 V)

VNEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . −10 V to −60 V VCC (externally supplied) . . . . . . . . . . . . . . . . VNEG + 9.5 V to VNEG + 12.0 V
VCC (internally regulated) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VNEG + 8.5 V Digital Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V to VCC

RECOMMENDED EXTERNAL COMPONENTS ( SEE TYPICAL APPLICATIONS CIRCUIT )

L = 68 mH, COUT = 220 mF // 0.1 mF, CIN = 33 mF, CBYPASS = 0.1 mF, CVCC = 1 mF, RSENSE = 0.25 W, 0.5 W

SPECIFICATIONSa (All Voltages Are With Respect To VNEG Unless Otherwise Specified)
Test Conditions (Internally Regulated) Limits
Unless Otherwise Specified
p −40 to 85_C

Parameter Symbol VNEG = −10 to −60 V Tempb Minc Typd Maxc Unit
Output Voltage (with respect to GND = 0 V)
+5-V Converter Full 4.80 5.00 5.20
VOUT 10 mA <ILOAD < 250 mA V
+3.3-V Converter Full 3.17 3.30 3.43

Line Regulation (with respect to GND = 0 V)

Line Regulation −60 V v VNEG v −40 V Full 1 %

VCC (Internal Regulator)

VCC Bias Voltage VCC Full 7.5 8.5 9.5 V

UVLO
VCC −
Under Voltage Lockout Turn-On Full 6.6 7.6 8.7
VNEG V
Hysteresis DV Room 0.6

Soft-Start
Error Amplifier Start-Up Current ISS VOUT = 0 V Room 10 mA

Oscillator
Switching Frequency fOSC Room 80 95 110 kHz

Error Amplifier
Transconductance gm Room 10 15 20 umho
Internal Error Amplifier
Clamp Voltage VCL Room 3.5 V
Output Clamp Voltage

Current Limit
Threshold Voltage VCS Full 0.57 0.67 0.77 V

MOSFET Switch
N-Channel MOSFET rDS(on) Room 1.5 2.5 W

www.vishay.com Document Number: 71112

2 S-40708—Rev. C, 19-Apr-04
 Si9121
Vishay Siliconix

SPECIFICATIONSa (All Voltages Are With Respect To VNEG Unless Otherwise Specified)
Test Conditions (Internally Regulated) Limits
Unless Otherwise Specified
p −40 to 85_C

Parameter Symbol VNEG = −10 to −60 V Tempb Minc Typd Maxc Unit
Supply
Supply Current
IGND GND to VNEG Full 1.2 1.5
(Internally Regulater)
Supply Current
ICC VCC to VNEG +10 V; VNEG >−20 V Full 1.5 2.0 mA
(External VCC Applied)
VOUT supply Current IOUT VOUT to VNEG Full 0.2 0.3
Start-Up Current ISTART VCC = 0 V Full 5 30

Thermal Shutdown
Thermal Shutdown Temperature TOTP 170
_C
Thermal Hysteresis THYS 25

Efficiency
+5 V Room 77
Efficiency 400 mA Output
400-mA Output, VNEG = −48
48 V %
+3.3 V Room 73

Notes
a. Refer to PROCESS OPTION FLOWCHART for additional information.
b. Room = 25_C, Full = as determined by the operating temperature suffix.
c. The algebraic convention whereby the most negative value is a minimum and the most positive a maximum.
d. Typical values are for DESIGN AID ONLY, not guaranteed nor subject to production testing.

DETAILED BLOCK DIAGRAM

4
5 COMP
VOUT
10 mA 2 mA
R1* VCC VNEG

− + Soft Start
GM H
+ 3.5 V −

R2*
+ VREF − Hiccup Discharge Mode
L
− 1.25 V 1.5 V +
6
GND
OSC OTP
7
VCC 8
0.6 V 50% Max. LX
− PWM Duty Cycle
−
+

Low Side +
Error Amp
R Q −VIN

OCL
3 Bias/
BYPASS Reference +
Circuit
0.67 V −

1
CS
−
− +
0.6-V Hysteresis
+
+ *R1 and R2 are internal voltage setting resistors used to set
2 8.5 V −
output voltage to fixed 3.3 V or 5 V.
VNEG

Document Number: 71112 www.vishay.com

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Si9121
Vishay Siliconix

TYPICAL CHARACTERISTICS (INTERNALLY REGULATED, 25_C UNLESS NOTED)

5-V VOUT vs. Temperature 3.3-V VOUT vs. Temperature

5.15 3.45

5.10 3.40

5.05 3.35
V OUT (V)

V OUT (V)
5.00 3.30

4.95 3.25

4.90 3.20

4.85 3.15
−40 −20 0 20 40 60 80 100 −40 −20 0 20 40 60 80 100
Temperature (_C) Temperature (_C)

rDS(on) vs. Temperature Frequency vs. Temperature

2.5 105
r DS(on) − On-Resistance ( W )

2.0 100
Frequency (kHz)

1.5 95

1.0 90

0.5 85
−40 −20 0 20 40 60 80 100 −40 −20 0 20 40 60 80 100
Temperature (_C) Temperature (_C)

Supply Current vs. VNEG

1.4
Supply Current (I GND )

1.3

25_C

1.2
85_C
−40_C

1.1

1.0
−60 −50 −40 −30 −20 −10
VNEG − (V)

www.vishay.com Document Number: 71112

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 Si9121
Vishay Siliconix

TYPICAL CHARACTERISTICS (INTERNALLY REGULATED, 25_C UNLESS NOTED)

Output Load vs. Efficiency (Si9121DY-3) Output Load vs. Efficiency (Si9121DY-5)
No VCC Winding No VCC Winding
90 90

80 VIN = 10 V 80 10 VIN

70 70

60 60
Efficiency (%)

48 VIN

Efficiency (%)
60 VIN
VIN = 48 V 50
50
VIN = 60 V
40 40

30 30

20 20

10 10

0 0
10 100 1000 10 100 1000

IOUT (mA) IOUT (mA)

Output Load vs. Efficiency (Si9121DY-3) Output Load vs. Efficiency (Si9121DY-5)
With VCC Winding With VCC Winding
90 90
48 VIN
VNEG = −10 V 80
80 10 VIN
70 70
60 VIN
60 60
Efficiency (%)

VNEG = −48 V
Efficiency (%)

50 50

40 VNEG = −60 V 40

30 30

20 20

10 10

0 0
10 100 1000 10 100 1000
IOUT (mA)
IOUT (mA)

Document Number: 71112 www.vishay.com

S-40708—Rev. C, 19-Apr-04 5
Si9121
Vishay Siliconix

PIN CONFIGURATION AND ORDERING INFORMATION

ORDERING INFORMATION
SOIC-8 Part Number Temperature Range Package
CS 1 8 LX Si9121DY-5-T1
Si9121DY-5-T1—E3
VNEG 2 7 VCC
Si9121 Tape and Reel
Si9121DY-3-T1
BYPASS 3 6 GND
−40
40 to 85_C
Si9121DY-3-T1—E3
COMP 4 5 VOUT
Si9121DY-5
Bulk
Top View Si9121DY-3

Eval Kit Temperature Range Board Type

Si9121DB-5 Surface Mount and
−10
10 to 70_C
Si9121DB-3 Thru-Hole

PIN DESCRIPTION
Pin Number Name Function
1 CS Current sense pin to detect the inductor current for current mode control and over current protection
2 VNEG Negative supply voltage (−10 V to −60 V)
3 BYPASS +1.5-V bandgap reference. Decouple with 0.1 mF capacitor.
4 COMP Compensation node to stabilize the converter
5 VOUT Output voltage feedback connected to the PWM summing comparator
6 GND Low impedance system ground
Internally generated supply voltage for the internal circuit and MOSFET drive circuit. Decouple with an external
7 VCC
bypass capacitor.
8 LX Inductor connection node

DETAILED DESCRIPTION
Start-Up the UVLO threshold. For the recommended frequency
compensation components (see Typical Application Circuit)
The UVLO circuit prevents the internal circuits from turning on the soft-start time is approximately 10 ms.
if VCC is less than 7.6 V (typical) above the negative supply Oscillator
voltage at VNEG pin. With a typical hysteresis of 0.6 V, the
controller is continuously powered on until the VCC voltage The oscillator is designed to operate at a nominal frequency
drops below 7.0 V. This hysteresis prevents the converter of 95 kHz with no external components. The 95-kHz operating
from oscillating during the start-up phase and unintentionally frequency allows the converter to operate in PWM mode
locking up the system. Once (VCC − VNEG) exceeds the UVLO during the full load condition even though the duty cycle is very
threshold the internal reference, oscillator, and soft-start low. The 95kHz switching frequency also allows the converter
circuits are enabled. to operate at optimal efficiency without a large output inductor
and capacitor.
Soft-Start
PWM Mode and Current Limit

The Si9121 has an on-chip soft-start circuit which utilizes the The Si9121 is a current-mode converter designed to operate
error amplifier external compensation network to ramp the in PWM mode. It features pulse-by-pulse peak current limiting
output NMOS transistor current limit which, in turn, allows the such that when the peak current sensed voltage on the CS pin
output voltage to rise gradually without excessive overshoot. is greater than 0.67 V the switch is turned off for the remainder
The soft-start circuit is enable once the VCC voltage exceeds of the clock cycle.

www.vishay.com Document Number: 71112

6 S-40708—Rev. C, 19-Apr-04
 Si9121
Vishay Siliconix

DETAILED DESCRIPTION

Bypass dissipation the regulator’s thermal circuit will continue to pulse

the regulator on and off. This is called thermal cycling.
The bypass voltage of the Si9121 is set at a particular positive
reference relative to the VNEG pin. The bypass voltage is used Hiccup Mode Short Circuit Protection
to set an accurate voltage and bias current for the on-chip
oscillator and soft-start circuits. The 0.1-mF ceramic In addition to the thermal shutdown and the cycle-by-cycle
decoupling capacitor is recommended between the bypass current limiting features already described, the Si9121 has a
and VNEG. No other components should be connected to this built-in hiccup mode timer to handle a continuous output
pin. short-circuit and to automatically restart the device when the
short-circuit is removed.
Error Amplifier
If an output short-circuit occurs, the device immediately enters
With a −48-V bus voltage, the converter is referenced to the a cycle-by-cycle current limiting mode. As long as the thermal
−48-V (VNEG) node as its system ground. However, the +5-V shutdown is not activated then the Si9121 automatically
or +3.3-V output is referenced to the GND pin, which is determines whether the internal hiccup timer should be started
normally connected to 0 V. To regulate this output voltage, the by monitoring the COMP pin. If VCOMP exceeds an upper
Si9121 has an on-chip error amplifier which continuously threshold voltage (approximately 3.5 V) the timer is started
monitors the output voltage and compares it to a reference and the external network at the COMP pin is discharged by an
voltage. This difference signal is level-shifted to the low side internal 2-mA current sink until VCOMP reaches approximately
circuit to control the power switch duty-cycle and hence the 1.5 V. (Note: all voltages are with respect to VNEG). At this
regulation of the output voltage. Frequency compensation for point, the circuit reverts to the normal soft-start mode, whereby
the error amplifier is achieved by connecting an external the COMP network is charged by its internal soft-start 10-mA
network between the COMP pin and the VNEG pin. current source and the circuit will attempt to start up in the
normal manner. However, if the output short circuit is still
VCC Regulator present, the converter will again enter the cycle-by-cycle
current limiting mode until the COMP pin voltage reaches
VCC is an internally generated bias supply voltage which 3.5 V, whereupon this cycle repeats until the short circuit is
should be externally bypassed with a 0.1-mF capacitor removed.
connected to the negative supply voltage, VNEG. No load
current should be drawn from the VCC pin. VCC may be The duty cycle imposed by the hiccup timer allows the Si9121
supplied from an external source of 9.5-V to 12-V referenced to handle continuous short-circuit without damage as long as
to VNEG. In this configuration, the internal VCC regulator is the recommended component values shown in the Typical
disabled when the VCC receives 9.5-V, or greater, above VNEG. Application Circuit are used.

Thermal Shutdown
MOSFET Switch
The Si9121 also includes thermal shutdown which shuts down
the device when junction temperature exceeds 170_C due to The low-side n-channel MOSFET switch is integrated to
over heating. In thermal shutdown once the die temperature provide optimum performance and to minimize the overall
cools to below 145_C the regulator is enabled. If the die converter size. The typical 1.5-W rDS(on) of the MOSFET
temperature is excessive due to high package power allows the converter to deliver up to 2 W of output power.

Document Number: 71112 www.vishay.com

S-40708—Rev. C, 19-Apr-04 7
Si9121
Vishay Siliconix

TYPICAL APPLICATION CIRCUIT

GND

L1
68 mH 220 mF 0.1 mF*
C2 C3
+

6 8 VOUT
GND LX +5 V/400 mA
3 5 10MQ100N or
BYPASS VOUT +3.3 V/400 mA
7 4
VCC COMP
2 1
VNEG CS R2
+ 75 kW
33 mF 0.1 mF 0.1 mF 1 mF 180 pF (27 kW for Si9121DY-3)
80 V C5* C1 C6 Si9121DY-5 R1 C7
C4 or 0.25 W +
1/ W
Si9121DY-3 2 0.1 mF
C8

VNEG

−10 to −60 V *Optional

FIGURE 1. Typical Applications Circuit
Silk Screen

T1
GND GND

68 mH Ns C7
C1 + Np C8*
220 mF 0.1 mF
33 mF
10 V +
80 V C2*
0.1 mF
D2
BAS21
D1 +VOUT
1 8 10MQ100N
CS COIL
2 7
VNEG VCC +5 V
3 6 @ 400 mA
BYPASS GND
4 5
COMP VOUT

C5
R1 C6
0.1 mF Si9121DY-5
0.25 C3 C4 1 mF
1/ W 0.1 mF 180 pF
2

R2
75 kW *Optional
(27 kW for Si9121DY-3)
−VIN

−10 to −60 V
FIGURE 2. Si9121 Application with External Vcc Through Winding

www.vishay.com Document Number: 71112

8 S-40708—Rev. C, 19-Apr-04
 Package Information
Vishay Siliconix

SOIC (NARROW): 8-LEAD

JEDEC Part Number: MS-012

8 7 6 5

E H

1 2 3 4

D h x 45
C
0.25 mm (Gage Plane)
A
All Leads

q 0.101 mm
e B A1 L
0.004"

MILLIMETERS INCHES
DIM Min Max Min Max
A 1.35 1.75 0.053 0.069
A1 0.10 0.20 0.004 0.008
B 0.35 0.51 0.014 0.020
C 0.19 0.25 0.0075 0.010
D 4.80 5.00 0.189 0.196
E 3.80 4.00 0.150 0.157
e 1.27 BSC 0.050 BSC
H 5.80 6.20 0.228 0.244
h 0.25 0.50 0.010 0.020
L 0.50 0.93 0.020 0.037
q 0° 8° 0° 8°
S 0.44 0.64 0.018 0.026
ECN: C-06527-Rev. I, 11-Sep-06
DWG: 5498

Document Number: 71192 www.vishay.com

11-Sep-06 1
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