Datasheet

FB
TPS61170
R2
R1
VOUT
R1 Vout
Vout = 1.229 V x + 1 R1 = R2 x 1
R2 1.229 V
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V
1
VVV
1
(FL
1
I
ininfout
s
P
out
P
limin
max_out
V
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2
I
I(V
I
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TPS61170
www.ti.com SLVS789C – NOVEMBER 2007– REVISED APRIL 2011
APPLICATION INFORMATION
PROGRAM OUTPUT VOLTAGE
Figure 17. Program Output Voltage
To program the output voltage, select the values of R1 and R2 (See Figure 17) according to Equation 2.
(2)
Considering the leakage current through the resistor divider and noise decoupling to FB pin, an optimum value
for R2 is around 10k. The output voltage tolerance depends on the accuracy of the reference voltage and the
tolerance of R1 and R2.
MAXIMUM OUTPUT CURRENT
The overcurrent limit in a boost converter limits the maximum input current, and thus the maximum input power
for a given input voltage. The maximum output power is less than the maximum input power due to power
conversion losses. Therefore, the current-limit setting, input voltage, output voltage and efficiency can all affect
the maximum output current. The current limit clamps the peak inductor current; therefore, the ripple must be
subtracted to derive the maximum DC current. The ripple current is a function of the switching frequency,
inductor value and duty cycle. The following equations take into account of all the above factors for maximum
output current calculation.
(3)
where:
I
P
= inductor peak to peak ripple current
L = inductor value
V
f
= Schottky diode forward voltage
F
s
= switching frequency
V
out
= output voltage
(4)
where:
I
out_max
= Maximum output current of the boost converter
I
lim
= overcurrent limit
η = efficiency
For instance, when V
in
is 5 V, V
out
is 12 V, the inductor is 10 μH, the Schottky forward voltage is 0.2 V; and then
the maximum output current is 300 mA in a typical operation.
Copyright © 2007–2011, Texas Instruments Incorporated 15