Datasheet

IN(min) est IN(min) LIM est
O
UT(max)
OUT OUT
V η V I (1 %RPL/2) η
I = =
V V
´ ´ ´ - ´
R1
V
out = 1.229 V + 1
R2
Vout
R1 = R2 1
1.229V
æ ö
´
ç ÷
è ø
æ ö
´ -
ç ÷
è ø
( )
OUT IN out
o
ut
OUT ripple
V V I
C =
V Fs V
-
´ ´
TPS61175
www.ti.com SLVS892B –DECEMBER 2008–REVISED FEBRUARY 2012
(6)
where
I
LIM
= over current limit
η
est
= efficiency estimate based on similar applications or computed above
For instance, when VIN = 12-V is boosted to VOUT = 24-V, the inductor is 10-uH, the Schottky forward voltage is
0.4-V and the switching frequency is 1.2-MHz; then the maximum output current is 1.2-A in typical condition,
assuming 90% efficiency and a %RPL = 20%.
SETTING OUTPUT VOLTAGE
To set the output voltage in either DCM or CCM, select the values of R1 and R2 according to the following
equation.
(7)
Considering the leakage current through the resistor divider and noise decoupling into FB pin, an optimum value
for R2 is around 10k. The output voltage tolerance depends on the V
FB
accuracy and the tolerance of R1 and
R2.
SETTING THE SWITCHING FREQUENCY
Choose the appropriate resistor from the resistance versus frequency table Table 1 or graph Figure 13. A resistor
must be placed from the FREQ pin to ground, even if an external oscillation is applied for synchronization.
Increasing switching frequency reduces the value of external capacitors and inductors, but also reduces the
power conversion efficiency. The user should set the frequency for the minimum tolerable efficiency.
SETTING THE SOFT START TIME
Choose the appropriate capacitor from the soft start table Table 2. Increasing the soft start time reduces the
overshoot during start-up.
SELECTING THE SCHOTTKY DIODE
The high switching frequency of the TPS61175 demands a high-speed rectification for optimum efficiency.
Ensure that the diode’s average and peak current rating exceed the average output current and peak inductor
current. In addition, the diode’s reverse breakdown voltage must exceed the switch FET rating voltage of 40V.
So, the VISHAY SS3P6L-E3/86A is recommended for TPS61175. The power dissipation of the diode's package
must be larger than I
OUT(max)
x V
D
SELECTING THE INPUT AND OUTPUT CAPACITORS
The output capacitor is mainly selected to meet the requirements for the output ripple and load transient. Then
the loop is compensated for the output capacitor selected. The output ripple voltage is related to the capacitor’s
capacitance and its equivalent series resistance (ESR). Assuming a capacitor with zero ESR, the minimum
capacitance needed for a given ripple can be calculated by
(8)
where, Vripple= peak to peak output ripple. The additional output ripple component caused by ESR is calculated
using:
V
ripple_ESR
= I × R
ESR
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