Datasheet
12
LTC3832/LTC3832-1
sn3832 3832fs
In many applications, V
CC
can be powered from V
IN
through an RC filter. This supply can be as low as 3V. The
low quiescent current (typically 800µA) allows the use of
relatively large filter resistors and correspondingly small
filter capacitors. 100Ω and 4.7µF usually provide ad-
equate filtering for V
CC
. For best performance, connect the
4.7µF bypass capacitor as close to the LTC3832 V
CC
pin as
possible.
Gate drive for the top N-channel MOSFET Q1 is supplied
from PV
CC1
. This supply must be above V
IN
(the main
power supply input) by at least one power MOSFET V
GS(ON)
for efficient operation. An internal level shifter allows PV
CC1
to operate at voltages above V
CC
and V
IN
, up to 14V maxi-
mum. This higher voltage can be supplied with a separate
supply, or it can be generated using a charge pump.
Gate drive for the bottom MOSFET Q2 is provided through
PV
CC2
for the LTC3832 or V
CC
/PV
CC2
for the LTC3832-1.
This supply only needs to be above the power MOSFET
V
GS(ON)
for efficient operation. PV
CC2
can also be driven
from the same supply/charge pump for the PV
CC1
, or it can
be connected to a lower supply to improve efficiency.
APPLICATIO S I FOR ATIO
WUUU
Figure 8. Tripling Charge Pump
LTC3832
3832 F08
+
D
Z
12V
1N5242
10µF
G1
G2
0.1µF
Q1
L
O
Q2 C
OUT
V
OUT
0.1µF
PV
CC2
1N5817
1N5817
1N5817
PV
CC1
V
IN
3832 F6
+
V
CC
PV
CC2
PV
CC1
V
IN
G1
Q1
C
OUT
V
OUT
Q2
L
O
G2
INTERNAL
CIRCUITRY
LTC3832
3832 F7
+
V
CC
/PV
CC2
PV
CC1
V
IN
G1
Q1
C
OUT
V
OUT
Q2
L
O
G2
INTERNAL
CIRCUITRY
LTC3832-1
Figure 6. LTC3832 Power Supplies
Figure 7. LTC3832-1 Power Supplies
Figure 8 shows a tripling charge pump circuit that can be
used to provide 2V
IN
and 3V
IN
gate drive for the external
top and bottom MOSFETs respectively. These should fully
enhance MOSFETs with 5V logic level thresholds. This
circuit provides 3V
IN
– 3V
F
to PV
CC1
while Q1 is ON and
2V
IN
– 2V
F
to PV
CC2
where V
F
is the forward voltage of the
Schottky diodes. The circuit requires the use of Schottky
diodes to minimize forward drop across the diodes at
start-up. The tripling charge pump circuit can rectify any
ringing at the drain of Q2 and provide more than 3V
IN
at
PV
CC1
; a 12V zener diode should be included from PV
CC1
to PGND to prevent transients from damaging the circuitry
at PV
CC1
or the gate of Q1.
The charge pump capacitors for PV
CC1
refresh when the
G2 pin goes high and the switch node is pulled low by Q2.
The G2 on-time becomes narrow when LTC3832/
LTC3832-1 operates at a maximum duty cycle (95%
typical), which can occur if the input supply rises more
slowly than the soft-start capacitor or if the input voltage
droops during load transients. If the G2 on-time gets so
narrow that the switch node fails to pull completely to
ground, the charge pump voltage may collapse or fail to
start, causing excessive dissipation in external MOSFET,
Q1. This condition is most likely with low V
CC
voltages and
high switching frequencies, coupled with large external
MOSFETs which slow the G2 and switch node slew rates.
The LTC3832/LTC3832-1 overcome this problem by sens-
ing the PV
CC1
voltage when G1 is high. If PV
CC1
is less than
2.5V above V
CC
, the maximum G1 duty cycle is reduced to
70% by clamping the COMP pin at 1.8V (QC in the Block