Datasheet

1
IN
S
V
D
V
h×
= -
( )
(max) (min)
1
2
L
OUT LIM
I
I I D
D
æ ö
= - × -
ç ÷
è ø
( )
2 1
OUT
L
SW peak
I
I
I
D
D
= +
-
IN
L
S
V D
I
f L
×
D =
×
TPS61085
www.ti.com SLVS859A –JUNE 2008–REVISED APRIL 2012
Design Procedure
The first step in the design procedure is to verify that the maximum possible output current of the boost converter
supports the specific application requirements. A simple approach is to estimate the converter efficiency, by
taking the efficiency numbers from the provided efficiency curves or to use a worst case assumption for the
expected efficiency, e.g. 90%.
1. Duty cycle, D:
(1)
2. Maximum output current, I
OUT(max)
:
(2)
3. Peak switch current in application, I
SW(peak)
:
(3)
with the inductor peak-to-peak ripple current, ΔI
L
(4)
and
V
IN
Minimum input voltage
V
S
Output voltage
I
LIM(min)
Converter switch current limit (minimum switch current limit = 3.2 A)
f
S
Converter switching frequency (typically 1.2 MHz or 650 kHz)
L Selected inductor value
η Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation)
The peak switch current is the steady state peak switch current that the integrated switch, inductor and external
Schottky diode has to be able to handle. The calculation must be done for the minimum input voltage where the
peak switch current is the highest.
Soft-start
The boost converter has an adjustable soft-start to prevent high inrush current during start-up. To minimize the
inrush current during start-up an external capacitor connected to the soft-start pin SS and charged with a
constant current, is used to slowly ramp up the internal current limit of the boost converter when charged with a
constant current. When the EN pin is pulled high, the soft-start capacitor C
SS
is immediately charged to 0.3 V.
The capacitor is then charged at a constant current of 10 μA typically until the output of the boost converter V
S
has reached its Power Good threshold (roughly 98% of V
S
nominal value). During this time, the SS voltage
directly controls the peak inductor current, starting with 0 A at V
SS
= 0.3 V up to the full current limit at V
SS
=0.8
V. The maximum load current is available after the soft-start is completed. The larger the capacitor the slower the
ramp of the current limit and the longer the soft-start time. A 100 nF capacitor is usually sufficient for most of the
applications. When the EN pin is pulled low, the soft-start capacitor is discharged to ground.
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