Datasheet

LT3991/LT3991-3.3/LT3991-5
21
3991fa
Typical applicaTions
4V Step-Down Converter with a High Impedance Input Source
10pF
SW
FB
SS
RT
V
IN
48V
V
OUT
4V
1.2A*
4.7µF
C
BULK
100µF
0.47µF
100µF
412k
118k
f = 400kHz
10µH
1M
GND
BD
SYNC
10M
100k
LT3991
3991 TA07a
EN BOOST
PG
2nF
432k
+
–
+
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
WHERE V IS VOLTAGE OF SOURCE, R IS
INTERNAL SOURCE IMPEDANCE, AND η IS
LT3971 EFFICIENCY. MAXIMUM OUTPUT
CURRENT OF 1.2A CAN BE SUPPLIED FOR A
SHORT TIME BASED ON THE ENERGY
WHICH CAN BE SOURCED BY THE BULK
INPUT CAPACITANCE.
P
POUT(MAX) = • η
OUT(MAX)
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
POUT(MAX) = • η
= •
POUT(MAX) = • η
POUT(MAX) = • η
η
POUT(MAX) = • η
V
POUT(MAX) = • η
2
POUT(MAX) = • η
4R
POUT(MAX) = • η
POUT(MAX) = • η
R
500µs/DIV
3991 TA07b
V
IN
10V/DIV
V
OUT
200mV/DIV
I
L
1A/DIV
Sourcing a Maximum Load Pulse Start-Up from High Impedance Input Source
2ms/DIV
3991 TA07c
V
IN
2V/DIV
V
OUT
2V/DIV
I
L
1A/DIV