ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 9

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

PIN FUNCTIONS
PLLIN/MODE (Pin 1/Pin 3): External Synchronization
Input to Phase Detector and Forced Continuous Mode
Input. When an external clock is applied to this pin, the
phase-locked loop will force the rising TG signal to be
synchronized with the rising edge of the external clock,
and the regulator operates in forced continuous mode.
When not synchronizing to an external clock, this input
determines how the LTC3891 operates at light loads. Pull-
ing this pin to ground selects Burst Mode operation. An
internal 100k resistor to ground also invokes Burst Mode
operation when the pin is floated. Tying this pin to INTV
forces continuous inductor current operation. Tying this
pin to a voltage greater than 1.2V and less than INTV
–1.3V selects pulse-skipping operation.
SGND (Pins 2, 3, Exposed Pad Pin 21/Pins 4, 5,
Exposed Pad Pin 21): Small-signal ground, must be routed
separately from high current grounds to the common
(–) terminals of the C
both be electrically connected to small signal ground for
proper operation.The exposed pad must be soldered to
PCB ground for rated thermal performance.
RUN (Pin 4/Pin 6): Digital Run Control Input. Forcing this
pin below 1.16V shuts down the controller. Forcing this
pin below 0.7V shuts down the entire LTC3891, reducing
quiescent current to approximately 14µA.
SENSE
Current Comparator. When greater than INTV
SENSE
SENSE
current comparator is normally connected to DCR sensing
network or current sensing resistor. The ITH pin voltage and
controlled offsets between the SENSE
conjunction with R
V
back voltage from an external resistive divider across
the output.
ITH (Pin 8/Pin 10): Error Amplifier Outputs and Switching
Regulator Compensation Point. The current comparator
trip point increases with this control voltage.
FB
(Pin 7/Pin 9): Receives the remotely sensed feed-
–
–
+
pin supplies power to the current comparator.
(Pin 5/Pin 7): The (–) Input to the Differential
(Pin 6/Pin 8): The (+) input to the differential
SENSE
IN
set the current trip threshold.
capacitor. Pins 2, 3/4, 5, must
(QFN/eTSSOP)
–
and SENSE
CC
– 0.5V, the
+
pins in
CC
CC
PGOOD (Pin 9/Pin 11): Open-Drain Logic Output. PGOOD
is pulled to ground when the voltage on the V
within 10% of its set point.
TG (Pin 10/Pin 12): High Current Gate Drives for Top
N-channel MOSFET. This is the output of floating driver
with a voltage swing equal to INTV
the switch node voltage SW.
SW (Pin 11/ Pin 13): Switch Node Connection to Induc-
tor.
BOOST (Pin 12/Pin 14): Bootstrapped Supply to the Top-
side Floating Driver. A capacitor is connected between the
BOOST and SW pin and a Schottky diode is tied between
the BOOST and INTV
pin is from INTV
BG (Pin 13/Pin 15): High Current Gate Drive for Bottom
(Synchronous) N-channel MOSFET. Voltage swing at this
pin is from ground to INTV
INTV
Low Dropout Regulator. The driver and control circuits
are powered from this voltage source. Must be decoupled
to PGND with a minimum of 2.2µF ceramic or other low
ESR capacitor. Do not use the INTV
purpose.
EXTV
Internal LDO Connected to INTV
INTV
V
Connection in the Applications Information section. Do
not float or exceed 14V on this pin.
PGND (Pin 16/Pin 18): Driver Power Ground. Connects to
the source of bottom (synchronous) N-channel MOSFET
and the (–) terminal of C
V
should be tied between this pin and the SGND pins.
I
Range Inputs. Tying this pin to SGND, FLOAT or INTV
sets the maximum current sense threshold to one of three
different levels for the comparator.
LIM
IN
IN
(Pin 17/Pin 19): Main Supply Pin. A bypass capacitor
whenever EXTV
(Pin 18/Pin 20): Current Comparator Sense Voltage
CC
CC
CC
power, bypassing the internal LDO powered from
(Pin 14/Pin 16): Output of the Internal Linear
(Pin 15/Pin 17): External Power Input to an
CC
to (V
CC
CC
pins. Voltage swing at the BOOST
is higher than 4.7V. See EXTV
IN
IN
.
+ INTV
CC
.
CC
CC
CC
).
. This LDO supplies
CC
superimposed on
LTC3891
pin for any other
FB
pin is not
9
3891f
CC
CC