LTC1929-PG LINER [Linear Technology], LTC1929-PG Datasheet - Page 11

• Manufacturer Part Number: LTC1929-PG
• Description: 2-Phase, High Efficiency,Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

OPERATIO
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
(LTC1929 only) allows selection of internal, precision feed-
back resistors for high common mode rejection differencing
applications, or direct access to the actual amplifier inputs
without these internal feedback resistors for other applica-
tions. The AMPMD pin is grounded to connect the internal
precision resistors in a unity-gain differencing application
(default for the LTC1929-PG), or tied to the INTV
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 am-
plifier has an output slew rate of 5V/ s and is capable of
driving capacitive loads with an output RMS current typi-
cally up to 25mA. The amplifier is not capable of sinking
current and therefore must be resistively loaded to do so.
Power Good (PGOOD) Pin (LTC1929-PG Only)
The PGOOD pin is connected to an open drain of a
MOSFET. The MOSFET turns on and pulls the pin low when
the output is not within 7.5% of its nominal output level
as determined by its resistive feedback divider. When the
APPLICATIO S I FOR ATIO
The basic LTC1929 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.
Finally, C
ripple current (that PolyPhase
C
ripple voltage and load step specifications (also minimized
with PolyPhase). Current mode architecture provides in-
herent current sharing between output stages. The circuit
shown in Figure 1 can be configured for operation up to an
input voltage of 28V (limited by the external MOSFETs).
OUT
SENSE1, 2
is chosen with low enough ESR to meet the output
IN
. Once R
is selected for its ability to handle the input
U
SENSE1, 2
U
(Refer to Functional Diagram)
OUT
U
are known, L1 and L2 can be
+
TM
and V
operation minimizes) and
OUT
W
–
benefits regula-
U
CC
pin to
output meets the 7.5% requirement, the MOSFET is
turned off within 10 s and the pin is allowed to be pulled
up by an external source.
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 current >5 A 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 condi-
tion. 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.
R
R
current. The LTC1929 current comparator has a maxi-
mum threshold of 75mV/R
mode range of SGND to 1.1( INTV
parator threshold sets the peak inductor current, yielding
a maximum average output current I
value less half the peak-to-peak ripple current, I
Allowing a margin for variations in the LTC1929 and
external component values yields:
PolyPhase is a trademark of Linear Technology Corporation.
SENSE
SENSE1, 2
R
SENSE
Selection For Output Current
= 2(50mV/I
are chosen based on the required output
LTC1929/LTC1929-PG
MAX
)
SENSE
and an input common
CC
MAX
). The current com-
equal to the peak
11
L
.