ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 11

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
Feedback Amplifier
Each side of the LTC1702A senses the output voltage at
V
gram). This is a real op amp with a low impedance output,
85dB open-loop gain and 25MHz gain-bandwidth product.
The positive input is connected internally to an 800mV
reference, while the negative input is connected to the FB
pin. The output is connected to COMP, which is in turn
connected to the soft-start circuitry and from there to the
PWM generator.
Unlike many regulators that use a resistor divider con-
nected to a high impedance feedback input, the LTC1702A
is designed to use an inverting summing amplifier topol-
ogy with the FB pin configured as a virtual ground. This
allows flexibility in choosing pole and zero locations not
available with simple g
allows the use of “type 3” compensation, which provides
a phase boost at the LC pole frequency and significantly
improves loop phase margin (see Figure 3). The Feedback
Loop/Compensation section contains a detailed explana-
tion of type 3 feedback loops.
OUT
with an internal feedback op amp (see Block Dia-
U
m
INFORMATION
configurations. In particular, it
U
W
COMP
R2
Figure 3. “Type 3” Feedback Loop
U
FB
C2
+
–
0.8V
C1
FB
PGOOD Flags
PGOOD is an open-drain output, allowing it to be wire-
ORed with other open-drain/open-collector signals. An
external pull-up resistor is required for PGOOD to swing
high. Any time the FB pin is more than 5% below the
programmed value for more than 100µs, PGOOD will pull
low, indicating that the output is out of regulation. PGOOD
remains active during soft-start and current limit. The
100µs delay ensures that short output transient glitches
that are successfully “caught” by the PGOOD comparator
don’t cause momentary glitches at the PGOOD pin.
When either side of the LTC1702A is in shutdown, its
associated PGOOD pin will go high. This behavior allows
a valid PGOOD reading when the two PGOOD pins are tied
together, even if one side is shut down. It also reduces
quiescent current by eliminating the excess current drawn
by the pull-up at the PGOOD pin. As soon as the RUN/SS
pin rises above the shutdown threshold and the side
comes out of shutdown, the PGOOD pin will pull low until
the output voltage is valid. If both sides are shut down at
the same time, both PGOOD pins will go high. To avoid
confusion, if either side of the LTC1702A is shut down, the
host system should ignore the associated PGOOD pin.
R
B
R3
R1
C3
1702A F03
V
OUT
LTC1702A
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