Datasheet

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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)
Choose the next lower standard value : R
(ISENSE)
= 120 m. Verify the inductor current ripple after the inductor
has been determined in step 5.
If the r
DS(on)
of the PMOS is used to sense the inductor current, a PMOS with less than 138 m must be used
for the application.
2. Calculate the resistors for the output voltage divider using V
O
= 3.3 V and V
FB
= 1.21 V
R1 + R2
ǒ
V
O
V
FB
Ǔ
–R2 + 1.72 R2
(19)
Choose R2 = 360 k, and then get R1 = 619 k. Select the next standard value: R1 = 620 k
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
resistor is used. Otherwise a PMOS with less r
DS(on)
must be selected.
Verify the maximum power dissipation of the PMOS using:
P
(cond)
+
ǒ
I
O
Ǔ
2
r
DS(on)
+ (0.5 A)
2
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
ǒ
1–
V
O
V
I
Ǔ
+ I
O
ǒ
1–
3.3 V
4.2 V
Ǔ
+ 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 × I
O
for the application. For the inductor, a series resistance of 100 m is assumed.
For minimum-off-time, the inductor value is:
L +
ǒ
V
O
) V
(SCHOTTKY)
) R
RL
I
O
Ǔ
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).