ADP1621ARMZ-R7 datasheet(22/32 Pages) AD

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ADP1621

Data Sheet

Rev. D | Page 22 of 32

EXAMPLES OF APPLICATION CIRCUITS

STANDARD BOOST CONVERTER—DESIGN

EXAMPLE

The example covered here is for the ADP1621 configured as a

standard boost converter, as shown in Figure 33, where lossless

current sensing is employed. The design parameters are VIN =

3.3 V, VOUT = 5 V, and a maximum load current of 1 A.

To begin this design, a switching frequency of 600 kHz is chosen

(by setting RFREQ to 32 kΩ, see Figure 30) so that a small inductor

and small output capacitors can be used. The duty cycle is cal-

culated from Equation 1 to be 0.4, given a forward-voltage drop of

0.5 V for the Schottky diode. The feedback resistors are calculated

to be R1 = 35.7 kΩ and R2 = 11.5 kΩ from Equation 4.

Assuming that the inductor ripple is 30% of 1/(1 − D) times

the maximum load current, the inductor size is calculated to be

about 4.4 µH, according to Equation 9. The small, magnetically

shielded 4.7 µH Toko FDV0630-4R7M inductor is selected.

Because ceramic capacitors have very low ESR (a few milliohms),

a 47 µF/6.3 V Murata GRM31CR60J476M ceramic capacitor is

chosen for the input capacitor. The output voltage ripple for a

given COUT, ESR, and ESL can be found by solving Equation 12.

By choosing an output voltage ripple equal to 1% of the output

voltage, Equation 12 yields that the minimum COUT required is

100 µF and the maximum ESR required is 25 mΩ. Other com-

binations of capacitance and ESR are possible by choosing a

much larger COUT and a larger ESR. In this case, a small 1 µF

ceramic capacitor and two 150 µF Sanyo POSCAP™ capacitors

are selected. The low ESR ceramic capacitor helps to suppress

the high frequency overshoot at the output. POSCAP has low

ESR and high capacitance in a relatively small package. Ceramic

capacitors can also be used. Generally, bigger ceramic capacitors

are more expensive.

The next step is to choose a Schottky diode. The average

and rms diode currents are calculated to be 1.0 A and 1.3 A,

respectively, using Equations 14 and 15. A Vishay SSA33L

Schottky diode meets the current and thermal requirements

and is an excellent choice.

The power MOSFET must be chosen based on threshold voltage

(VT), on resistance (RDSON), maximum voltage and current ratings,

and gate charge. The rms current through the MOSFET is given

by Equation 18 as 1.1 A. The Vishay Si7882DP is a 20 V n-channel

power MOSFET that meets the current and thermal requirements.

It comes in a PowerPAK® package and offers low RDSON and gate

charge. At VGS = 2.5 V, the on resistance, RDSON, is 8 mΩ.

The loop-compensation components are chosen to be RCOMP =

9.1 kΩ and CCOMP = 1.7 nF from Equations 30 and 31, respectively.

A roll-off capacitor of C2 = 120 pF is also added. The slope-

compensation resistor is set to be RS = 80 Ω from Equation 34.

Lastly, given the chosen components, the peak inductor current

as set by the current limit circuitry is given by Equation 35 as

IL,PK = 12 A. Thus, the maximum load current, assuming CCM

operation, is given by Equation 36 as ILOAD,MAX = 8 A, which is

safely above the 1.0 A load current requirement for this design

example. Note that the current limit is a strong function of RCS,

which can vary device to device and with temperature. In addition,

note that RCS can be implemented with an external current-sense

resistor or with the RDSON of a MOSFET. Variations in RCS and the

other parameters in Equations 35 and 36 must be taken into

account if precise current limiting is necessary. Due to the parasitic

resistance of PCB traces, RS might need to be adjusted on the

actual circuit board to achieve the desired current limit. Keep in

mind that RS must be less than 1.6 kΩ. Using a MOSFET with a

different RDSON or adjusting RCS can also set the current limit to

the desired level.

ADP1621

GATE

PGND

AGND

SDSN

COMP

FREQ

GND

35.7kΩ

11.5kΩ

47µF

6.3V

4.7µH

CCOMP

1.8nF

COUT1

1µF

10V

fOSC = 600kHz

C1 = MURATA GRM31CR60J476M

COUT3 = SANYO POSCAP 6TPE150M

L1 = TOKO FDV0630-4R7M

M1 = VISHAY Si7882DP

D1 = VISHAY SSA33L

80Ω

COUT2

10µF

10V

RFREQ

31.6kΩ

COUT3

150µF

6.3V

×2

1µF

10V

0.1µF

10V

120pF

RCOMP

9.09kΩ

VOUT = 5V

VIN = 3.3V

Figure 33. Typical Boost Converter Application Circuit

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