LT3991/LT3991-3.3/LT3991-5
19
3991fa
applicaTions inForMaTion
5V Step-Down Converter
3.3V Step-Down Converter 5V Step-Down Converter
SW
FB
SS
RT
V
IN
IN
V
5V
1.2A
4.7µF
0.47µF
47µF
10pF
309k
118k
f = 400kHz
10µH
1M
GND
BD
SYNC
OFF ON
LT3991
3991 TA02
EN BOOST
PG
Typical applicaTions
loss. The die temperature is calculated by multiplying the
LT3991 power dissipation by the thermal resistance from
junction to ambient.
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem
-
perature. When the power switch is closed, the power
Schottky
diode
is in parallel with the power converter’s
output filter stage. As a result, an increase in a diode’s
leakage current results in an effective increase in the load,
and a corresponding increase in input power. Therefore,
the catch Schottky diode must be selected with care to
avoid excessive increase in light load supply current at
high temperatures.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
2.5V Step-Down Converter
10pF
SW
FB
SS
RT
V
IN
IN
V
1.2A
4.7µF
1µF
47µF
909k
162k
f = 300kHz
10µH
1M
GND
SYNC
OFF ON
LT3991
3991 TA03
EN BOOST
PG
BD
SW
FB
SS
RT
V
IN
IN
V
1.2A
4.7µF
0.47µF
47µF
1M
118k
10µH
1.78M
GND
f = 400kHz
BD
SYNC
OFF ON
10pF
LT3991
3991 TA09
EN BOOST
PG
SW
V
OUT
SS
RT
V
IN
IN
V
5V
1.2A
4.7µF
0.47µF
47µF
118k
f = 400kHz
15µH
GND
BD
SYNC
OFF ON
LT3991-5
3991 TA10
EN./UVLO BOOST
PG
