LTC3830-1 Linear Technology, LTC3830-1 Datasheet - Page 12

• Manufacturer Part Number: LTC3830-1
• Description: High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation
• Manufacturer: Linear Technology

LTC3830/LTC3830-1
APPLICATIO S I FOR ATIO
filter capacitors. 100
equate filtering for V
4.7 F bypass capacitor as close to the LTC3830 V
possible.
Gate drive for the top N-channel MOSFET Q1 is supplied
from PV
power supply input) by at least one power MOSFET V
for efficient operation. An internal level shifter allows PV
to operate at voltages above V
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
LTC3830-1 and the 8-lead LTC3830. This supply only
needs to be above the power MOSFET V
operation. PV
charge pump for the PV
lower supply to improve efficiency.
Figure 8 shows a tripling charge pump circuit that can be
used to provide 2V
top and bottom MOSFETs respectively. These should fully
enhance MOSFETs with 5V logic level thresholds. This
circuit provides 3V
2V
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
12
IN
CC2
D
12V
1N5242
10 F
– 2V
Z
LTC3830
for the 16-lead LTC3830 or V
CC1
F
PV
to PV
1N5817
. This supply must be above V
CC2
CC2
Figure 8. Tripling Charge Pump
CC2
can also be driven from the same supply/
PV
1N5817
IN
IN
CC1
U
CC
where V
G1
G2
– 3V
and 3V
. For best performance, connect the
CC1
and 4.7 F usually provide ad-
U
F
0.1 F
, or it can be connected to a
to PV
F
IN
is the forward voltage of the
CC
gate drive for the external
1N5817
and V
0.1 F
CC1
W
V
while Q1 is ON and
IN
IN
Q1
Q2
GS(ON)
CC
, up to 14V maxi-
/PV
L
O
IN
CC2
for efficient
U
(the main
+
CC
for the
GS(ON)
3830 F08
pin as
C
OUT
CC1
V
OUT
ringing at the drain of Q2 and provide more than 3V
PV
to PGND to prevent transients from damaging the circuitry
at PV
Care should be taken when using a charge pump to power
PV
than 4V) or high switching frequencies. 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 maximum 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
switching frequencies, coupled with large external
MOSFETs which slow the G2 and switch node slew rates.
Workarounds include:
• Increasing the soft-start capacitor to limit the duty cycle
• Using smaller MOSFETs with lower gate capacitance
• Using an external higher voltage supply to power PV
Another alternative is to add an external circuit to limit the
duty cycle when PV
charge pump is not running, PV
equal to V
about (V
which limits the duty cycle to about 50% and allows the
charge pump to start up. Once PV
(V
the limit on duty cycle is removed.
For applications with a 5V or higher V
be tied to V
supplied using a doubling charge pump as shown in Figure
9a. This circuit provides 2V
CC
at start up
(where possible) to reduce the G2 rise/fall time and
switch node slew rates
if available
CC1
CC1
+ V
CC1
; a 12V zener diode should be included from PV
in applications with low V
TQ3
CC
or the gate of Q1.
CC
/6 + V
IN
), the voltage at the soft-start pin goes high and
if a logic level MOSFET is used. PV
and the voltage at the soft-start pin will be
BE
). This is about 1.2V with a V
CC1
is low, as shown in Figure 9b. If the
IN
– V
F
CC
CC1
to PV
CC
CC1
supply voltages (less
IN
will be less than or
CC1
voltages and high
supply, PV
rises higher than
while Q1 is ON.
CC
CC1
sn3830 3830fs
of 3.3V,
CC2
can be
IN
can
CC1
CC1
at