LTC3831 LINER [Linear Technology], LTC3831 Datasheet - Page 11

• Manufacturer Part Number: LTC3831
• Description: High Power Synchronous Switching Regulator Controller for DDR Memory Termination
• Manufacturer: LINER [Linear Technology]

SYNC METHOD
APPLICATIO S I FOR ATIO
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
for the main power input, PV
gate drive and a clean, low ripple V
internal circuitry (Figure 5).
In many applications, V
through an RC filter. This supply can be as low as 3V. The
low quiescent current (typically 800 A) allows the use of
relatively large filter resistors and correspondingly small
filter capacitors. 100
equate filtering for V
4.7 F bypass capacitor as close to the LTC3831 V
possible.
TERMINATION
UNDER SYNC
TRADITIONAL
KEEPS RAMP
WITH EARLY
AMPLITUDE
CONSTANT
LTC3831
RAMP
RAMP AMPLITUDE
Figure 4. External Synchronization Operation
SHDN
FREE RUNNING
ADJUSTED
RAMP SIGNAL
200kHz
U
CC
. For best performance, connect the
and 4.7 F usually provide ad-
CC
U
can be powered from V
CC1
and PV
W
CC
for the LTC3831
CC2
WITH EXT SYNC
RAMP SIGNAL
for MOSFET
U
CC
pin as
3831 F04
IN
IN
Gate drive for the top N-channel MOSFET Q1 is supplied
from PV
power supply input) by at least one power MOSFET
V
allows PV
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
MOSFET V
driven from the same supply/charge pump for the PV
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
consists of a Schottky diode from V
capacitor from PV
GS(ON)
USE FOR V
LTC3831
CC2
D
12V
1N5242
OPTIONAL
Z
CIRCUITRY
INTERNAL
. This supply only need to be above the power
V
CC1
for efficient operation. An internal level shifter
IN
CC
CC1
GS(ON)
LTC3831
7V
. This supply must be above V
to operate at voltages above V
Figure 6. Doubling Charge Pump
PV
CC2
for efficient operation. PV
PV
CC1
Figure 5. Supplies Input
IN
CC2
gate drive for Q1. The charge pump
to the switching node at the drain of
PV
CC1
PV
TG
BG
CC1
MBR0530T1
BG
0.1 F
TG
IN
V
to PV
IN
V
Q1
Q2
IN
Q1
Q2
LTC3831
L
CC1
CC2
O
CC
L
O
IN
and a 0.1 F
and V
can also be
+
(the main
+
3831 F05
11
C
OUT
3831 F06a
IN
C
V
OUT
CC1
, up
OUT
3831f
V
OUT
,