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LTC3831 Datasheet(PDF) 8 Page - Linear Technology

Part # LTC3831
Description  High Power Synchronous Switching Regulator Controller for DDR Memory Termination
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3831 Datasheet(HTML) 8 Page - Linear Technology

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LTC3831
8
3831fb
TEST CIRCUITS
Figure 2
SS
FREQSET
FB
COMP
R+
NC
NC
VFB
VCOMP
2.5V
TG
BG
SHDN VCC
VSHDN VCC
PVCC2 PVCC1
PVCC
IFB
6800pF
0.1μF
10μF
TG RISE/FALL
BG RISE/FALL
6800pF
3831 F02
R
IMAX
GND
LTC3831
PGND
+
APPLICATIONS INFORMATION
OVERVIEW
The LTC3831 is a voltage mode feedback, synchronous
switching regulator controller (see Block Diagram) de-
signed for use in high to medium power, DDR memory
termination. It includes an onboard PWM generator, a
ratiometric reference, two high power MOSFET gate
drivers and all necessary feedback and control circuitry
to form a complete switching regulator circuit. The PWM
loop nominally runs at 200kHz.
The LTC3831 is designed to generate an output voltage
that tracks at 1/2 of the external voltage connected be-
tween the R+ and Rpins. The LTC3831 can be used to
generate the termination voltage, VTT, for interface like
the SSTL_2 where VTT is a ratio of the interface supply
voltage, VDDQ. It is a requirement in the SSTL_2 interface
standard for VTT to track the interface supply voltage to
improve noise immunity. Using the LTC3831 to supply the
interface termination voltage allows large current sourc-
ing and sinking through the termination resistors during
bus transitions.
The LTC3831 includes a current limit sensing circuit that
uses the topside external N-channel power MOSFET as
a current sensing element, eliminating the need for an
external sense resistor. Also included is an internal soft-
start feature that requires only a single external capacitor
to operate. In addition, the part features an adjustable
oscillator which can free run or synchronize to an external
signal with frequencies from 100kHz to 500kHz, allowing
added flexibility in external component selection.
THEORY OF OPERATION
Primary Feedback Loop
The LTC3831 senses the output voltage of the circuit
through the FB pin and feeds this voltage back to the
internal transconductance error amplifier, ERR. The er-
ror amplifier compares the output voltage to the internal
ratiometric reference, VREF, and outputs an error signal to
the PWM comparator. VREF is set to 0.5 multiplied by the
voltage difference between the R+ and Rpins, using an
internal resistor divider.
This error signal is compared with a fixed frequency
ramp waveform, from the internal oscillator, to generate
a pulse width modulated signal. This PWM signal drives
the external MOSFETs through the TG and BG pins. The
resulting chopped waveform is filtered by LO and COUT
which closes the loop. Loop compensation is achieved
with an external compensation network at the COMP pin,
the output node of the error amplifier.
MIN, MAX Feedback Loops
Two additional comparators in the feedback loop provide
high speed output voltage correction in situations where
the error amplifier may not respond quickly enough. MIN
compares the feedback signal to a voltage 3% below VREF.
If the signal is below the comparator threshold, the MIN
comparator overrides the error amplifier and forces the
loop to maximum duty cycle, >91%. Similarly, the MAX
comparator forces the output to 0% duty cycle if the feed-


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