Datasheet

10
LTC1530
1530fa
Figure 5b plots the minimum required R
IMAX
resistor (kΩ)
versus the maximum operating load current (I
LMAX
=
I
LOAD
+ I
RIPPLE
/2) as a function of Q1’s R
DS(ON)
. Note that
during an intial power-up sequence (V
OUT
= 0V), the
inductor’s start-up current I
ST
is much higher than the
steady-state condition, I
LMAX
. The difference between I
ST
and I
LMAX
is affected by the input power supply slew rate,
the input and output voltages, the LTC1530 soft-start slew
rate, the maximum duty cycle and the inductor and output
capacitor values.
For a given application, the input and output requirements
are known and determine the main inductor and output
capacitor values. These values establish the transient load
recovery time. In general, a low value inductor combined
with high value output capacitance has a short transient
load recovery time at the expense of higher inductor ripple
and start-up current (I
RIPPLE
and I
ST
). However, if a small
inductor and large value output capacitors are chosen, the
value of R
IMAX
obtained from Figure 5b may be too small
to allow proper regulator start-up.
During start-up, if I
ST
is higher than the current limit
threshold set by the R
IMAX
resistor, the LTC1530 current
limit comparator turns on. This comparator then limits
input charging current by reducing duty cycle. During this
time, if V
OUT
doesn’t increase above one-half of the rated
value, the LTC1530 hard current limit circuit turns on. This
circuit forces the LTC1530 to repeat a soft-start cycle and
the power supply fails to start. If V
OUT
increases above
one-half of the rated value, the power supply output may
start-up properly depending on whether the limited input
current charges the output capacitor and prevents hard
current limit action.
Therefore, select R
IMAX
with the start-up current (I
ST
) in
mind. Choosing R
IMAX
to set the current comparator
threshold above I
ST
ensures proper power supply start-up
as well as recovery from an output fault condition.
Figures 6a and 6b plot the start-up I
ST
vs output capaci-
tance and inductance for unloaded and loaded conditions
with the current limit circuit disabled. Figures 6a and 6b
are provided as examples. Actual I
ST
under start-up con-
ditions must be measured for any application circuit so
that R
IMAX
can be properly chosen.
I
LMAX
(A)
0
MINIMUM REQUIRED R
IMAX
(Ω)
5500
4500
3500
2500
1500
500
16 18
1530 F05b
426
810 14
12
20
R
IMAX
≥ 500Ω
I
LMAX
= I
LOAD
+ I
RIPPLE
/2
Q1 R
DS(ON)
= 0.05Ω
0.04Ω
0.03Ω
0.02Ω
0.01Ω
Figure 5b. Minimum Required R
IMAX
vs I
LMAX
OUTPUT CAPACITANCE (mF)
0
START-UP I
ST
(A)
25
20
15
10
5
0
2
468
1530 F06a
10 12
T
A
= 25°C
V
IN
= 5V
I
LOAD
= 0A
L = 1.2µH
L = 4.7µH
L = 2.4µH
Figure 6a. Start-Up I
ST
vs Output Capacitance
OUTPUT CAPACITANCE (mF)
0
START-UP I
ST
(A)
30
25
20
15
10
5
0
2
468
1530 F06b
10 12
T
A
= 25°C
V
IN
= 5V
I
LOAD
= 10A
L = 1.2µH
L = 4.7µH
L = 2.4µH
Figure 6b. Start-Up I
ST
vs Output Capacitance
APPLICATIO S I FOR ATIO
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