Datasheet
LTC3557/LTC3557-1
21
35571fc
APPLICATIONS INFORMATION
External HV Buck Control Through the V
C
Pin
The WALL, ACPR and V
C
pins can be used in conjunction
with an external high voltage buck regulator such as the
LT
®
3480, LT3481 or LT3505 to provide power directly
to the V
OUT
pin through a power P-channel MOSFET as
shown in Figures 3-5 (consult the factory for a complete
list of approved high voltage buck regulators). When the
WALL pin voltage exceeds 4.3V, V
C
pin control circuitry
is enabled and drives the V
C
pin of the LT3480, LT3481 or
LT3505. The V
C
pin control circuitry is designed so that no
compensation components are required on the V
C
node.
The voltage at the V
OUT
pin is regulated to the larger of
(BAT + 300mV) or 3.6V as shown in Figures 6 and 7. The
feedback network of the high voltage regulator should be
set to generate an output voltage higher than 4.4V (be sure
to include the output voltage tolerance of the buck regula-
tor). The V
C
control of the LTC3557 overdrives the local
V
C
control of the external high voltage buck. Therefore,
once the V
C
control is enabled, the output voltage is set
independent of the buck regulator feedback network.
V
IN
2
3
DFLS240L
0.47µF
22µF
C
OUT
V
OUT
UP TO
2A
BAT
35571 F03
Li-Ion
Si2333DS
Si2333DS
(OPT)
6.8µH
4
5
6
7
NC
NC
10
BOOST
V
C
26 3 25
23
21
22
LTC3557
LTC3557-1
WALL ACPR
GATE
V
OUT
BAT
RUN/SS
150k
40.2k
68nF
4.7µF
HV
IN
8V TO 38V
(TRANSIENTS
TO 60V)
499k
100k
SW
R
T
1
BD
8
11 9
FB
LT3480
LT3480
HIGH VOLTAGE
BUCK CIRCUITRY
GND V
C
+
V
IN
2
3
7
DFLS240L
0.47µF
22µF
C
OUT
UP TO
2A
V
OUT
BAT
35571 F04
Li-Ion
Si2333DS
Si2333DS
(OPT)
6.8µH
4
5
6
NC
10
BOOST
V
C
26 3 25
23
21
22
LTC3557
LTC3557-1
WALL ACPR
GATE
V
OUT
BAT
RUN/SS
150k
60.4k
68nF
4.7µF
HV
IN
8V TO 34V
549k
200k
SW
R
T
BIAS
1
BD
8
11 9
FB
LT3481
LT3481
HIGH VOLTAGE
BUCK CIRCUITRY
GND V
C
+
Figure 3. LT3480 Buck Control Using V
C
(800kHz Switching)
Figure 4. LT3481 Buck Control Using V
C
(800kHz Switching)