LTC3455_15 Datasheet, PDF (26/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
TYPICAL APPLICATIONS
USB 5V
1Ω
C6
4.7μF
8
6
5.6V
USB
CONTROLLER
5
WALL 5V
1Ω
C7
4.7μF
10
D1
C5
10μF
3.32k
1k
4
11
C8, 0.1μF
1.24k
3
2
SINGLE
CELL Li-ION
+
3.3V TO 4.2V
2.49k
9
C4
4.7μF
21
USB
MODE
SUSPEND
15
HSON
19
USBHP
ON2
22
μC
PWRON
RST ²⁰
V_MAX
23
PBSTAT
LTC3455/LTC3455-1
CHRG
ON
WALLFB
HSO
TIMER
PROG
AO
V_BAT
HSI
SW2
AI
FB2
1M 1M
1.8V
24
ON/OFF
14
3.3V, HS
C3
1μF
V_MAX
2k
17
13
12 ^{L2, 4.7μH}
100pF
16
18
M1
FDN304P
OR
Si2305DS
249k
2.49k
3.3V
1.2A
C2
2x10μF
80.6k
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
7 ^{L1, 4.7μH}
SW1
10pF
1
FB1
25
GND
3455 F17
1.8V
100k ^0.4A
C1
10μF
80.6k
Figure 17. LTC3455/LTC3455-1 Application with 3.3V Output Current Increased to 1.2A
Increasing 3.3V Output Current to 1.2A
With an internal current limit of 900mA, Switcher 2 typically
provides a 3.3V, 600mA output. While this output current
is sufficient for many portable devices, some applications
need a 3.3V supply capable of providing more than 1A.
Figure 17 shows how to implement a higher current 3.3V
output using the LTC3455/LTC3455-1. By adding one tiny
SOT23 PMOS and using the AI/AO amplifier as an LDO,
the 3.3V output now provides 1.2A of output current.
Switcher 2 is programmed for an output voltage of 3.3V,
and the LDO is programmed for an output voltage of 3.2V
(3% lower). As long as the load current is low enough for
Switcher 2 to provide, the LDO is turned off completely.
This circuit is ideal for applications that need the higher
3.3V output current for only a brief time. Switcher 2 will
normally provide all of the output current, and the LDO will
turn on briefly to provide the higher load currents.
26
When the load current exceeds what Switcher 2 can provide,
the 3.3V output droops slightly and the LDO provides the
additional current needed. Figure 18 shows the transient
response when the 3.3V output current is stepped from
0.5A to 1.2A. More output capacitance can be added to
improve the 3.3V transient response during these high
current load steps.
V_OUT2 (3.3V)
100mV/DIV
AC-COUPLED
I_OUT2
0.5A/DIV
0.5A TO 2A STEP
M1 GATE
2V/DIV
500μs/DIV
3455 F18
Figure 18. Load Current Step (0.5A to 1.2A) for 3.3V Output
3455fc