Techcode® DATASHEET

1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

General Description Features

The TD6817 is a high efficiency monolithic synchronous  High Efficiency: Up to 96%

buck regulator using a constant frequency, current mode  High Efficiency at light loads
architecture. The device is available in an adjustable  Very Low Quiescent Current: Only 20uA During
version and fixed output voltages of 1.5V and 1.8V. Operation
Supply current during operation is only 20mA and drops  2A Output Current
to ≤1mA in shutdown. The 2.5V to 5.5V input voltage  2.5V to 5.5V Input Voltage Range
range makes the TD6817 ideally suited for single Li-Ion  1.5MHz Constant Frequency Operation
battery-powered applications. 100% duty cycle provides  No Schottky Diode Required
low dropout operation, extending battery life in portable  Low Dropout Operation: 100% Duty Cycle
systems.Automatic Burst Mode operation increases  0.6V Reference Allows Low Output Voltages
efficiency at light loads, further extending battery life.  Shutdown Mode Draws ≤1uA Supply Current
Switching frequency is internally set at 1.5MHz, allowing  Current Mode Operation for Excellent Line and Load
the use of small surface mount inductors and capacitors. Transient Response
The internal synchronous switch increases efficiency and  Overtemperature Protected
eliminates the need for an external Schottky diode. Low  TSOT23-5 Package is Available
output voltages are easily supported with the 0.6V
feedback reference voltage. The TD6817 is available in
TSOT23-5 package. Applications

 Cellular Telephones
 Personal Information Appliances
 Wireless and DSL Modems
 Digital Still Cameras
 MP3 Players
 Portable Instruments

Package Types

TSOT23-5

Figure 1. Package Types of TD6817

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Pin Name Description

Pin Assignments
1 RUN Run Control Input. Forcing this pin
above 1.5V enables the part.
Forcing this pin below 0.3V shuts
down the device. In shutdown, all
functions are disabled drawing
<1A supply current. Do not leave
RUN floating.
2 GND Ground Pin.

3 SW Switch Node Connection to

Inductor. This pin connects to the
drains of the internal main and
synchronous power MOSFET
TSOT23-5
switches.
4 VIN Main Supply Pin. Must be closely
decoupled to GND, Pin 2, with a
2.2F or greater ceramic capacitor.
5 VFB Feedback Pin. Receives the
feedback voltage from an external
resistive divider across the output.
5 VOUT Output Voltage Feedback Pin. An
internal resistive divider divides the
output voltage down for comparison
to the internal reference voltage.

Ordering Information

TD6817 □ □

Circuit Type Output Versions

Blank：Adj
12：1.2V
Package 15：1.5V
T:TSOT23-5 18：1.8V

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Functional Block Diagram

Figure2:Functional Block Diagram of TD6817

Type Application Circuit

Figure 3. Type Application Circuit of TD6817

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Absolute Maximum Ratings

Note1: Stresses greater than those listed under Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation

of the device at these or any other conditions above those indicated in the operation is not implied. Exposure to absolute maximum rating conditions for extended

periods may affect reliability.

Parameter Value Unit

Input Supply Voltage -0.3 ~6 V

RUN, VFB Voltages -0.3 ~ VIN V

SW Voltage -0.3V ~(VIN+0.3) V

P-Channel Switch Source Current (DC) 2250 mA

N-Channel Switch Sink Current (DC) 2250 mA

Peak SW Sink and Source Current 2.5 A

Operating Temperature Range -40~+85 ºC

Junction Temperature 125 ºC

Lead Temperature (Soldering, 10 sec) 300 ºC

Storage Temperature Range -65~150 ºC

Electrical Characteristics

Unless otherwise specified, VIN= 3.6V TA=25 ºC.

Symbol Parameter Conditions Min. Typ. Max. Unit

IVFB Feedback Current nA

TA = 25C 0.5880 0.6000 0.6120

Regulated Feedback
VFB 0C TA 85C 0.5865 0.6000 0.6135 V
Voltage
–40C TA 85C 0.5850 0.6000 0.6150
Reference Voltage Line
VIN = 2.5V to 5.5V 0.04 0.4 %/ V
Regulation

Regulated Output TD6817-1.5, IOUT = 150mA 1.455 1.500 1.545 V

VOUT
Voltage TD6817-1.8, IOUT = 150mA 1.746 1.800 1.854
Output Voltage Line
VIN = 2.5V to 5.5V 0.04 0.4 %/ V
Regulation

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Electrical Characteristics(Cont.)

Unless otherwise specified, VIN= 3.6V TA=25 ºC.

Symbol Parameter Conditions Min. Typ. Max. Unit

VIN = 3V, VFB = 0.5V or
IPK Peak Inductor Current VOUT = 90%, Duty Cycle < 2.4 2.5 2.6 A
35%
Output Voltage Load
VLOADREG 0.5 %
Regulation
VIN Input Voltage Range 2.5 5.5 V

Input DC Bias Current

VFB = 0.5V or VOUT =
Active Mode 300 400 uA
90%, ILOAD = 0A
IS
VFB = 0.62V or VOUT =
Sleep Mode 20 35 uA
103%, ILOAD = 0A
Shutdown VRUN = 0V, VIN = 4.2V 0.1 1 uA
VFB = 0.6V or VOUT =
fOSC 1 1.5 1.8 MHz
Oscillator Frequency 100%
VFB = 0V or VOUT = 0V 400 KHz
RDS(ON) of P-Channel
RPFET ISW = 100mA 0.11 0.13 Ω
FET
RDS(ON) of N-Channel
RNFET ISW = -100mA 0.12 0.15 Ω
FET
VRUN = 0V, VSW = 0V or
ILSW SW Leakage 0.01 1 uA
5V, VIN = 5V
VRUN RUN Threshold 0.3 1 1.5 V

IRUN RUN Leakage Current 1 uA

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Typical Operating Characteristics

Reference Voltage Oscillator Frequency

Oscillator Frequency vs Supply Voltage RDS(ON) vs Temperature

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Typical Operating Characteristics(Cont.)

RDS(ON) vs Input Voltage Efficiency vs Output Current

Efficiency vs Output Current Efficiency vs Output Current

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Typical Operating Characteristics(Cont.)

Efficiency vs Output Current Output Voltage vs Output Current

Efficiency vs Input Voltage Dynamic Supply Current vs Supply Voltage

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Typical Operating Characteristics

P-FET Leakage vs Temperature N-FET Leakage vs Temperature

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description

the EA amplifier’s output rises above the sleep threshold

signaling the BURST comparator to trip and turn the top
Main Control Loop
MOSFET on. This process repeats at a rate that is
dependent on the load demand.
The TD6817 uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) Short-Circuit Protection
switches are internal. During normal operation, the
internal top power MOSFET is turned on each cycle When the output is shorted to ground, the frequency of
when the oscillator sets the RS latch, and turned off the oscillator is reduced to about 400kHz, 1/4 the
when the current comparator, ICOMP, resets the RS nominal frequency. This frequency foldback ensures that
latch. The peak inductor current at which ICOMP resets the inductor current has more time to decay, thereby
the RS latch, is controlled by preventing runaway. The oscillator’s frequency will
the output of error amplifier EA. When the load current progressively increase to 1.5MHz when VFB or VOUT
increases, it causes a slight decrease in the feedback rises above 0V.
voltage, FB, relative to the 0.6V reference, which in turn,
causes the EA amplifier’s output voltage to increase until
Dropout Operation
the average inductor current matches the new load
current. While the top MOSFET is off, the bottom
MOSFET is turned on until either the inductor current As the input supply voltage decreases to a value
starts to reverse, as indicated by the current reversal approaching the output voltage, the duty cycle increases
comparator IRCMP, or the beginning of the next clock toward the maximum on-time. Further reduction of the
cycle. supply voltage forces the main switch to remain on for
more than one cycle until it reaches 100% duty cycle.
The output voltage will then be determined by the input
Burst Mode Operation voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
The TD6817 is capable of Burst Mode operation in which An important detail to remember is that at low input
the internal power MOSFETs operate intermittently supply voltages, the RDS(ON) of the P-channel switch
based on load demand. increases (see Typical Performance Characteristics).
In Burst Mode operation, the peak current of the inductor Therefore, the user should calculate the power
is set to approximately 200mA regardless of the output dissipation when the TD6817 is used at 100% duty cycle
load. Each burst event can last from a few cycles at light with low input voltage (See Thermal Considerations in
loads to almost continuously cycling with short sleep the Applications Information section).
intervals at moderate loads. In between these burst
events, the power MOSFETs and any unneeded circuitry
are turned off, reducing the quiescent current to 20uA. In
this sleep state, the load current is being supplied solely
from the output capacitor. As the output voltage droops,

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description(Cont.)

The basic TD6817 application circuit is shown in Figure

3. External component selection is driven by the load
Low Supply Operation
requirement and begins with the selection of L followed
by CIN and COUT.
The TD6817 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction Inductor Selection
in the maximum output current as a function of input
voltage for various output voltages. For most applications, the value of the inductor will fall in
the range of 1uH to 4.7uH. Its value is chosen based on
the desired ripple current. Large value inductors lower
Slope Compensation and Inductor Peak
ripple current and small value inductors result in higher
ripple currents. Higher VIN or VOUT also increases the
Current
ripple current as shown in equation 1. A reasonable
starting point for setting ripple current is DIL = 800mA
Slope compensation provides stability in constant (40% of 2000mA).
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak The DC current rating of the inductor should be at least
current for duty cycles >40%. However, the TD6817 uses equal to the maximum load current plus half the ripple
a patent-pending scheme that counteracts this current to prevent core saturation. Thus, a 2200mA rated
compensating ramp, which allows the maximum inductor inductor should be enough for most applications
peak current to remain unaffected throughout all duty (2000mA + 200mA). For better efficiency, choose a low
cycles. DC-resistance
inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher DIL) will cause
this to occur at lower load currents, which can cause a
dip in efficiency in the upper range of low current
operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to increase.

Maximum Output Current vs Input Voltag

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description(Cont.)

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is
Inductor Core Selection
commonly used for design because even significant
deviations do not offer much relief. Note that the
Different core materials and shapes will change the capacitor manufacturer’s ripple current ratings are often
size/current and price/current relationship of an inductor. based on 2000 hours of life. This makes it advisable to
Toroid or shielded pot cores in ferrite or permalloy
further derate the capacitor, or choose a capacitor rated
materials are small and don’t radiate much energy, but at a higher temperature than required. Always consult
generally cost more than powdered iron core inductors the manufacturer if there is any question.
with similar electrical characteristics. The choice of which The selection of COUT is driven by the required effective
style inductor to use often depends more on the price vs
series resistance (ESR). Typically, once the ESR
size requirements and any radiated field/EMI
requirement for COUT has been met, the RMS current
requirements than on what the TD6817 requires to
rating generally far exceeds the IRIPPLE(P-P)
operate. requirement. The output ripple DVOUT is determined by:

CIN and COUT Selection

In continuous mode, the source current of the top

MOSFET is a square wave of duty cycle VOUT/VIN. To where f = operating frequency, COUT = output
prevent large voltage transients, a low ESR input capacitanceand DIL = ripple current in the inductor. For a
capacitor sized for the maximum RMS current must be fixed output voltage, the output ripple is highest at
used. The maximum RMS capacitor current is given by: maximum input voltage since DIL increases with input
voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the
case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. An
excellent choice is the AVX TPS series of surface mount
tantalum. These are specially constructed and tested for
low ESR so they give the lowest ESR for a given volume.
Other capacitor types include Sanyo POSCAP, Kemet
T510 and T495 series, and Sprague 593D and 595D
series. Consult the manufacturer for other specific
recommendations.

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description(Cont.)

Using Ceramic Input and Output

Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high
ripple current, high voltage rating and low ESR make Figure 4:Setting the output Voltage
them ideal for switching regulator applications. Because Vout R1 R2
the TD6817’s control loop does not depend on the output
1.2V 150K 150K
capacitor’s ESR for stable operation, ceramic capacitors
1.5V 160K 240K
can be used freely to achieve very low output ripple and
1.8V 150K 300K
small circuit size.
2.5V 150K 470K
However, care must be taken when ceramic capacitors
3.3V 150K 680K
are used at the input and the output. When a ceramic
capacitor is used at the input and the power is supplied Table 1. Vout VS. R1, R2, Cf Select Table
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop Efficiency Considerations
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at The efficiency of a switching regulator is equal to the
VIN, large enough to damage the part. output power divided by the input power times 100%. It is
When choosing the input and output ceramic capacitors, often useful to analyze individual losses to determine
choose the X5R or X7R dielectric formulations. These what is limiting the efficiency and which change would
dielectrics have the best temperature and voltage produce the most improvement. Efficiency can be
characteristics of all the ceramics for a given value and expressed as:
size. Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a
percentage of input power.
Output Voltage Programming
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
In the adjustable version, the output voltage is set by a losses in TD6817 circuits: VIN quiescent current and I2R
resistive divider according to the following formula: losses. The VIN quiescent current loss dominates the
efficiency loss at very low load currents whereas the I2R
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve
The external resistive divider is connected to the output, at very low load currents can be misleading since the
allowing remote voltage sensing as shown in Figure4. actual power lost is of no consequence as illustrated in
Figure 5.

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description(Cont.)

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs
can be obtained from the Typical Performance
Charateristics curves. Thus, to obtain I2R losses, simply
add RSW to RL and multiply the result by the square of
the average output current. Other losses including CIN
and COUT ESR dissipative losses and inductor core
losses generally account for less than 2% total additional
loss.

Thermal Considerations

Figure 4:Power Lost VS Load Current In most applications the TD6817 does not dissipate
1. The VIN quiescent current is due to two components: much heat due to its high efficiency. But, in applications
the DC bias current as given in the electrical where the TD6817 is running at high ambient
characteristics and the internal main switch and temperature with low supply voltage and high duty
synchronous switch gate charge currents. The gate cycles, such as in dropout, the heat dissipated may
charge current results from switching the gate exceed the maximum junction temperature of the part. If
capacitance of the internal power MOSFET switches. the junction temperature reaches approximately 150°C,
Each time the gate is switched from high to low to high both power switches will be turned off and the SW node
again, a packet of charge, dQ, moves from VIN to will become high impedance.
ground. The resulting dQ/dt is the current out of VIN that To avoid the TD6817 from exceeding the maximum
is typically larger than junction temperature, the user will need to do some
the DC bias current. In continuous mode, IGATECHG thermal analysis. The goal of the thermal analysis is to
=f(QT + QB) where QT and QB are the gate charges of determine whether the power dissipated exceeds the
the internal top and bottom switches. Both the DC bias maximum junction temperature of the part. The
and gate charge losses are proportional to VIN and temperature rise is given by:
thustheir effects will be more pronounced at higher TR = (PD)(qJA)
supply voltages. where PD is the power dissipated by the regulator and
2. I2R losses are calculated from the resistances of the qJA is the thermal resistance from the junction of the die
internal switches, RSW, and external inductor RL. In to the ambient temperature.
continuous mode, the average output current flowing The junction temperature, TJ, is given by:
through inductor L is “chopped” between the main switch TJ = TA + TR
and the synchronous switch. Thus, the series resistance where TA is the ambient temperature.
looking into the SW pin is a function of both top and
bottom MOSFET RDS(ON) and the duty cycle (DC) as
follows:

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Function Description(Cont.)

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an
amount equal to (ILOAD • ESR), where ESR is the
effective series resistance of COUT. ILOAD also begins
to charge or discharge COUT, which generates a
feedback error signal. The regulator loop then acts to
return VOUT to its steadystate value. During this
recovery time VOUT can be monitored for overshoot or
ringing that would indicate a stability problem. For a
detailed explanation of switching control loop theory.
A second, more severe transient is caused by switching
in loads with large (>1F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in
parallel with COUT, causing a rapid drop in VOUT. No
regulator can deliver enough current to prevent this
problem if the load switch resistance is low and it is
driven quickly. The only solution is to limit the rise time of
the switch drive so that the load rise time is limited to
approximately (25 • CLOAD).Thus, a 10F capacitor
charging to 3.3V would require a 250s rise time, limiting
the charging current to about 130mA.

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Package Information

TSOT23-5 Package Outline Dimensions

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1.5MHz 2A Synchronous Step-Down Regulator Dropout TD6817

Design Notes

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