LTC3831_15 Datasheet, PDF (11/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
APPLICATIONS INFORMATION
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
Figure 4. External Synchronization Operation
controller feedback loop. As a result, the loop crossover
frequency increases and it may cause the feedback 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
adequate filtering for V_CC. For best performance, connect
the 4.7μF bypass capacitor as close to the LTC3831 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 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
Q2. This circuit provides 2V_IN – V_F to PV_CC1 while Q1 is
ON and V_IN – V_F while Q1 is OFF where V_F is the forward
voltage of the Schottky diode. Ringing at the drain of Q2
can cause transients above 2V_IN at PV_CC1; if V_IN is higher
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
3831fb
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