LTC1629 LINER [Linear Technology], LTC1629 Datasheet - Page 11

• Manufacturer Part Number: LTC1629
• Description: PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

OPERATIO
INTV
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTV
EXTV
regulator supplies INTV
taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTV
This allows the INTV
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section. An external Schottky
diode can be used to minimize the voltage drop from
EXTV
the specified INTV
applied to EXTV
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both V
tion in high current applications and/or applications hav-
ing electrical interconnection losses. The AMPMD pin
(available on the LTC1629 only) allows selection of inter-
nal precision feedback resistors for high common mode
rejection differencing applications, or direct access to the
actual amplifier inputs without these internal feedback
resistors for other applications. The AMPMD pin is
grounded to connect the internal precision resistors in a
unity-gain differencing application or tied to the INTV
pin to bypass the internal resistors and make the amplifier
inputs directly available. The amplifier is a unity-gain
stable, 2MHz gain-bandwidth, >120dB open-loop gain
design. The amplifier has an output slew rate of 5V/ s and
is capable of driving capacitive loads with an output RMS
current typically up to 25mA. The amplifier is not capable
APPLICATIO S I FOR ATIO
The basic LTC1629 application circuit is shown in Figure 1
on the first page. External component selection is driven
by the load requirement, and begins with the selection of
R
chosen. Next, the power MOSFETs and D1 and D2 are
selected. The operating frequency and the inductor are
chosen based mainly on the amount of ripple current.
SENSE1, 2
CC
CC
CC
/EXTV
to INTV
pin is left open, an internal 5V low dropout
. Once R
CC
CC
CC
Power
U
CC
SENSE1, 2
for additional gate drive capability.
in applications requiring greater than
U
CC
current. Voltages up to 7V can be
(Refer to Functional Diagram)
power to be derived from a high
CC
OUT
U
power. If the EXTV
are known, L1 and L2 can be
+
and V
OUT
W
–
benefits regula-
CC
CC
. When the
to INTV
U
CC
pin is
CC
CC
.
of sinking current and therefore must be resistively loaded
to do so. The differential amplifier is configured as a unity-
gain differencing amplifier in the LTC1629-PG.
Power Good (PGOOD) (LTC1629-PG Only)
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET turns on when the output is not
within 7.5% of its nominal output level as determined by
the feedback divider. When the output is within 7.5% of
its nominal value, the MOSFET is turned off within 10 s
and the PGOOD pin should be pulled up by an external
resistor to a source of up to 7V.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition lasts for a long enough period
as determined by the size of the RUN/SS capacitor, the
controller will be shut down until the RUN/SS pin voltage
is recycled. This built-in latchoff can be overidden by
providing a >5 A pull-up current at a compliance of 5V to
the RUN/SS pin. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
condition. Foldback current limiting is activated when the
output voltage falls below 70% of its nominal level whether
or not the short-circuit latchoff circuit is enabled.
Finally, C
ripple current (that PolyPhase operation minimizes) and
C
ripple voltage and load step specifications (also minimized
with PolyPhase). The circuit shown in Figure 1 can be
configured for operation up to an input voltage of 28V
(limited by the external MOSFETs).
OUT
is chosen with low enough ESR to meet the output
IN
is selected for its ability to handle the input
LTC1629/LTC1629-PG
11