TPS64200,TPS64201

TPS64202,TPS64203

SLVS485 − AUGUST 2003

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17

DESIGN EXAMPLE FOR AN APPLICATION USING A LI-ION CELL (3.3 V TO 4.2 V) TO

GENERATE 3.3 V/500 mA

1.

Calculate the sense resistor for the current limit:

R

(ISENSE) v

V

(ISENSE)

min

1.3

I

O

+

90 mV

1.3

0.5 A

+ 138 mW

(18)

has been determined in step 5.

for the application.

2.

R1

+ R2

V

O

V

FB

–R2

+ 1.72

R2

(19)

Choose R2 = 360 k

Ω, and then get R1 = 619 kΩ. Select the next standard value: R1 = 620 kΩ

3.

Select the external PMOS

For a Li-Ion to 3.3-V conversion, the minimum input voltage is 3.3 V. Therefore, the converter runs in 100%

mode (duty cycle=1) and the maximum PMOS current is equal to the output current.

I

(PMOS) +

I

O +

0.5 A

(20)

The Si2301ADS is selected for this application because it meets the requirements when an external sense

Verify the maximum power dissipation of the PMOS using:

P

(cond) +

I

O

2

r

DS(on) +

0.19

W + 48 mW

(21)

4.

Select the external diode

For the Schottky diode, the worst case current is at high input voltage (4.2 V for a Li-Ion cell).

I

(diode)(Avg) [

I

O

1–

V

O

V

I

+ I

O

1– 3.3 V

4.2 V

+ 0.11 A

(22)

The MBR0530T1 is selected because it meets the voltage and current requirements. The forward voltage is

about 0.3 V. Do not use a Schottky diode which is much larger than required as it also typically has more leakage

current and capacitance which reduces efficiency.

5.

Calculate the inductor value.

If the output voltage is close to the input voltage, the switching frequency is determined by the minimum off-time.

Therefore, the TPS64202 is used for the maximum switching frequency possible. Allow an inductor ripple

current of 0.3

For minimum-off-time, the inductor value is:

L

+

V

O )

V

(SCHOTTKY) )

R

RL

I

O

t

off

,min

DI

+

(3.3 V

) 0.3 V ) 0.05 V)

0.3

ms

0.3

0.5 A

+ 7.3 mH

(23)

For a low inductor current ripple, select the next available larger inductor with L = 10

µH. This provides an

inductor ripple current of 110 mA (peak-to-peak).