Datasheet

12
LTC3830/LTC3830-1
3830fa
Input Supply Considerations/Charge Pump
The 16-lead LTC3830 requires four supply voltages to
operate: V
IN
for the main power input, PV
CC1
and PV
CC2
for
MOSFET gate drive and a clean, low ripple V
CC
for the
LTC3830 internal circuitry (Figure 6). The LTC3830-1 and
the 8-lead LTC3830 have the PV
CC2
and V
CC
pins tied
together inside the package (Figure 7). This pin, brought
out as V
CC
/PV
CC2
, has the same low ripple requirements
as the 16-lead part, but must also be able to supply the gate
drive current to Q2.
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 LTC3830 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 16-lead LTC3830 or V
CC
/PV
CC2
for the
LTC3830-1 and the 8-lead LTC3830. 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.
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 refresh when the G2 pin goes
high and the switch node is pulled low by Q2. The G2 on-
time becomes narrow when LTC3830 operates at maxi-
mum duty cycle (95% typical), which can occur if the input
supply rises more slowly than the soft-start capacitor or
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 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.
APPLICATIO S I FOR ATIO
WUUU
Figure 8. Tripling Charge Pump
LTC3830
3830 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