Datasheet

LT8641
11
Rev B
For more information www.analog.com
PIN FUNCTIONS
SYNC/MODE (Pin 17): This pin programs four different
operating modes: 1) Burst Mode. Tie this pin to ground
for Burst Mode operation at low output loads—this will
result in ultralow quiescent current. 2) Pulse-skipping
mode. Float this pin for pulse-skipping mode. This mode
offers full frequency operation down to low output loads
before pulse skipping occurs. When floating, pin leakage
currents should be <1µA. See Block Diagram for internal
pull-up and pull-down resistance. 3) Spread spectrum
mode. Tie this pin high to INTV
CC
(~3.4V) or an external
supply of 3V to 4V for pulse-skipping mode with spread
spectrum modulation. 4) Synchronization mode. Drive
this pin with a clock source to synchronize to an external
frequency. During synchronization the part will operate in
pulse-skipping mode.
GND (Pins 18): LT8641 Ground Pin. Connect this pin to
system ground and to the ground plane.
PG (Pin 19): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±8% of the final regulation voltage, and there are
no fault conditions. PG is valid when V
IN
is above 3.4V,
regardless of EN/UV pin state.
FB (Pin 20): The LT8641 regulates the FB pin to 0.8V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and V
OUT
.
Typically, this capacitor is 4.7pF to 22pF.
SW (Exposed Pad Pins 21, 22): The exposed pads should
be connected and soldered to the SW trace for good thermal
performance. If necessary due to manufacturing limita-
tions Pins 21 and 22 may be left disconnected, however
thermal performance will be degraded.
BLOCK DIAGRAM
+
+
–
+
–
SLOPE COMP
INTERNAL 0.8V REF
OSCILLATOR
200kHz TO 3MHz
BURST
DETECT
3.4V
REG
M1
M2
60k
600k
C
BST
C
OUT
V
OUT
8641 BD
SW
L
BST
8, 9, 21, 22
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±8%
V
C
INTV
CC
SHDN
THERMAL SHDN
INTV
CC
UVLO
V
IN
UVLO
SHDN
THERMAL SHDN
V
IN
UVLO
EN/UV
1V
+
–
14
4
3
18
GND
INTV
CC
2
BIAS
1
V
IN2
13
GND1
6, 7
GND2
10, 11
PG
19
FB
R1C1
R3
OPT
R4
OPT
R2
R
T
C
SS
OPT
V
OUT
20
TR/SS
1.9µA
16
RT
15
SYNC/MODE
17
V
IN1
V
IN
C
IN1
C
IN3
C
VCC
C
IN2
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