LTC3831 Datasheet, PDF (11/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
APPLICATIO S I FOR ATIO
SHDN
200kHz
FREE RUNNING
RAMP SIGNAL
TRADITIONAL
SYNC METHOD
WITH EARLY
RAMP
TERMINATION
RAMP SIGNAL
WITH EXT SYNC
RAMP AMPLITUDE
ADJUSTED
LTC3831
KEEPS RAMP
AMPLITUDE
CONSTANT
UNDER SYNC
3831 F04
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 maximum. 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. This supply only need 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 6 shows a doubling charge pump circuit that can be
used to provide 2V_IN gate drive for Q1. The charge pump
consists of a Schottky diode from V_IN to PV_CC1 and a 0.1µF
capacitor from PV_CC1 to the switching node at the drain of
Figure 4. External Synchronization Operation
decrease in ramp amplitude increases the open-loop gain
of the controller feedback loop. As a result, the loop
crossover frequency increases and it may cause the feed-
back loop to be unstable if the phase margin is insufficient.
To overcome this problem, the LTC3831 monitors the
peak voltage of the ramp signal and adjust the oscillator
charging current to maintain a constant ramp peak.
Input Supply Considerations/Charge Pump
The LTC3831 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 LTC3831
internal circuitry (Figure 5).
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 LTC3831 V_CC pin as
possible.
V_CC
PV_CC2 PV_CC1
V_IN
INTERNAL
CIRCUITRY
LTC3831
TG
Q1
L_O
V_OUT
BG
Q2
+
C_OUT
3831 F05
Figure 5. Supplies Input
OPTIONAL
USE FOR V_IN ≥ 7V
D_Z
12V
1N5242
PV_CC2
V_IN
MBR0530T1
PV_CC1
TG
0.1µF
Q1
L_O
BG
Q2
V_OUT
+
C_OUT
LTC3831
3831 F06a
Figure 6. Doubling Charge Pump
3831f
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