LTC3557_15 Datasheet, PDF (21/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
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.
HV_IN
8V TO 38V
(TRANSIENTS
TO 60V)
4.7μF
4
2
V_IN BOOST
68nF 150k
LT3480
5
3
RUN/SS SW
10
R_T
40.2k
7 _NC
6 _NC
1
BD ₈
FB
GND V_C
LT3480
HIGH VOLTAGE
BUCK CIRCUITRY
11 9
0.47μF
6.8μH
DFLS240L
26 3 25
V_C WALL ACPR
V_OUT
23
LTC3557
LTC3557-1
GATE
21
BAT ²²
499k
22μF
100k
Si2333DS
UP TO
2A
V_OUT
C_OUT
Si2333DS
(OPT)
+
BAT
Li-Ion
35571 F03
Figure 3. LT3480 Buck Control Using V_C (800kHz Switching)
HV_IN
8V TO 34V
4.7μF
4
2
V_IN BOOST
68nF 150k
5
LT3481
3
RUN/SS SW
10
7
R_T
BIAS
60.4k
6
NC
1
BD ₈
FB
GND V_C
LT3481
HIGH VOLTAGE
BUCK CIRCUITRY
11 9
0.47μF
6.8μH
DFLS240L
26 3 25
V_C WALL ACPR
V_OUT
23
LTC3557
LTC3557-1
GATE
21
BAT ²²
549k
22μF
200k
Si2333DS
UP TO
2A
V_OUT
C_OUT
Si2333DS
(OPT)
+
BAT
Li-Ion
35571 F04
Figure 4. LT3481 Buck Control Using V_C (800kHz Switching)
35571fc
21