Datasheet

LTC3891
10
3891fa
FUNCTIONAL DIAGRAM
PIN FUNCTIONS
TRACK/SS (Pin 19/Pin 1): External Tracking and Soft-
Start Input. The LTC3891 regulates the V
FB
voltage to the
smaller of 0.8V or the voltage on the TRACK/SS pin. An
internal 10μA pull-up current source is connected to this
pin. A capacitor to ground at this pin sets the ramp time
to final regulated output voltage. Alternatively, a resistor
divider on another voltage supply connected to this pin
allows the LTC3891 output to track another supply during
start-up.
SW
TOP
BOOST
TG
C
B
C
IN
D
D
B
PGND
BOT
BG
INTV
CC
INTV
CC
V
IN
C
OUT
V
OUT
3891 FD
R
SENSE
DROP
OUT
DET
BOT
TOP ON
S
R
Q
Q
SHDN
SLEEP
0.425V
ICMP
2.7V
0.65V
IR
2mV
SLOPE COMP
SENSE
+
SENSE
–
PGOOD
V
FB
0.88V
0.72V
L
+
–
+
–
FREQ
+
–
+
–
+
–
+
–
+
–
+
–
SWITCH
LOGIC
V
FB
R
A
C
C
R
C
C
C2
R
B
0.80V
TRACK/SS
0.88V
7µA
11V
RUN
ITH
TRACK/SS
+
–
C
SS
10µA
SHDN
CURRENT
LIMIT
FOLDBACK
SHDN
RST
2(V
FB
)
PLLIN/MODE
20µA
VCO
LDO
EN
INTV
CC
5.1V
SYNC
DET
100k
CLK2
CLK1
I
LIM
V
IN
EXTV
CC
LDO
PFD
EN
4.7V
5.1V
+
–
SGND
EA
OV
FREQ (Pin 20/Pin 2): The frequency control pin for the
internal VCO. Connecting the pin to GND forces the VCO
to a fixed low frequency of 350kHz. Connecting the pin
to INTV
CC
forces the VCO to a fixed high frequency of
535kHz. Other frequencies between 50kHz and 900kHz
can be programmed by using a resistor between FREQ
and GND. An internal 20µA pull-up current develops the
voltage to be used by the VCO to control the frequency.