LTC2923
6
2923fb
For more information www.linear.com/LTC2923
pin FuncTions
MS/DE Packages
VCC (Pin 1): Positive Supply Input Pin. The operating sup-
ply input range is 2.9V to 5.5V. An undervoltage lockout
circuit resets the part when the supply is below 2.5V. VCC
should be bypassed to GND with a 0.1µF capacitor.
ON (Pin 2): On Control Input. The ON pin has a threshold
of 1.23V with 75mV of hysteresis. An active high will cause
10µA to flow from the GATE pin, ramping up the supplies.
An active low pulls 10µA from the GATE pin, ramping the
supplies down. Pulling the ON pin below 0.4V resets the
electronic circuit breaker in the LTC2923. If a resistive
divider connected to VCC drives the ON pin, the supplies
will automatically start up when VCC is fully powered.
TRACK1, TRACK2 (Pins 3, 4): Tracking Control Input. A
resistive voltage divider between RAMPBUF and TRACKx
determines the tracking profile of a slave supply (see Ap-
plications Information). TRACKx pulls up to 0.8V and the
current supplied at TRACKx is mirrored at FBx. TRACKx
is capable of supplying at least 1mA when VCC = 2.9V.
Because a TRACKx pin is capable of supplying up to 30mA
under short-circuit conditions, avoid connecting TRACKx
to GND for extended periods. Limit the capacitance at
each TRACKx pin to less than 25pF. Float the TRACKx
pins if unused.
RAMPBUF (Pin 5): Ramp Buffer Output. Provides a low
impedance buffered version of the signal on the RAMP
pin. This buffered output drives the resistive dividers that
connect to the TRACKx pins. Limit the capacitance at the
RAMPBUF pin to less than 100pF.
GND (Pin 6): Circuit Ground.
Exposed Pad (Pin 13): The exposed pad may be left open
or connected to GND.
FB1, FB2 (Pins 8, 7): Feedback Control Output. FBx pulls
up on the feedback node of slave supplies. Tracking is
achieved by mirroring the current from TRACKx into FBx.
If the appropriate resistive divider connects RAMPBUF and
TRACKx, the FBx current will force OUTx to track RAMP. To
prevent damage to the slave supply, the FBx pin will not
force the slave’s feedback node above 1.7V. In addition,
it will not actively sink current from this node even when
the LTC2923 is unpowered. Float the FB pins if unused.
GATE (Pin 9/Pin 11): Gate Drive for External N-Channel FET.
When the ON pin is high, an internal 10µA current source
charges the gate of the external N-channel MOSFET. A
capacitor connected from GATE to GND sets the ramp rate.
An internal charge pump guarantees that GATE will pull up
to 5V above VCC ensuring that logic level N-channel FETs
are fully enhanced. When the ON pin is pulled low, the GATE
pin is pulled to GND with a 10µA current source. Under
a short-circuit condition, the electronic circuit breaker in
the LTC2923 pulls the GATE low immediately with 20mA.
Tie GATE to GND if unused. It is a good practice to add a
10Ω resistor between this capacitor and the FET’s gate
to prevent high frequency FET oscillations.
RAMP (Pin 10/Pin 12): Ramp Buffer Input. When the
RAMP pin is connected to the source of the external
N-channel FET, the slave supplies track the FET’s source
as it ramps up and down. If the GATE is fully enhanced
(GATE>RAMP+4.9V) and (VCC – RAMP > 200mV) indi-
cates a shorted output, then the electronic circuit breaker
trips and GATE quickly pulls low with 20mA. The GATE
will not ramp up again until ON is pulled below 0.4V and
then above 1.23V. Alternatively, when no external FET is
used, the RAMP pin can be tied directly to the GATE pin.
In this configuration, the supplies track the capacitor on
the GATE pin as it is charged and discharged by the 10µA
current source controlled by the ON pin. RAMP must not
be driven above VCC (except by the GATE pin).
SDO (Pin 9, DE Package Only): Slave Supply Shutdown
Output. SDO is an open-drain output that holds the shut-
down (RUN/SS) pins of the slave supplies low until the ON
pin is pulled above 1.23V. If the slave supply is capable
of operating with an input supply that is lower than the
LTC2923’s minimum operating voltage of 2.9V, the SDO
pin can be used to hold off the slave supplies. SDO will be
pulled low again when RAMP < 100mV and ON < 1.23V.
STATUS (Pin 10, DE Package Only): Power Good Status
Indicator. The STATUS pin is an open-drain output that
pulls low until GATE has been fully charged at which time
all supplies will have reached their final operating voltage.