LTC3734_15 Datasheet, PDF (11/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
OPERATION (Refer to Functional Diagram)
Operational Amplifier and Deep Sleep Offset
The internal operational amplifier provides a programmable
output offset at deep sleep state (when the STP_CPUB
signal is low). The offset percentage is programmed by
the resistor from RDPSLP to V_OA+ and the resistor from
output to V_OA+. The amplifier has an output slew rate of
5V/µs and is capable of driving capacitive loads with an
output RMS current typically up to 40mA. The open-loop
gain of the amplifier is >120dB and the unity-gain band-
width is 2MHz.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>10%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Power Good
The PGOOD pin is connected to the drain of an internal
N‑channel MOSFET. The MOSFET turns on when the out-
put voltage is not within ±10% of its nominal set point.
When the output voltage is within ±10% of its nominal set
point, the MOSFET turns off and PGOOD is high imped-
ance. PGOOD monitors the V_BOOT voltage when MCH_PG
is not asserted. During VID, deep sleep or deeper sleep
transitions, PGOOD is masked from going low for 110µs,
preventing the system from resetting during CPU mode
changes. When VID bits, STP_CPUB or DPRSLPVR signals
change again after a previous transition, but before the
timer expires, the internal timer resets.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the control-
ler has 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 out-
put voltage falls to less than 70% of its nominal output
voltage the RUN/SS capacitor begins discharging as-
suming 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 >5µA to the RUN/
SS pin. This current shortens the soft-start period but
also prevents net discharge of the RUN/SS capacitor dur-
ing 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.
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