Datasheet

LTC3455/LTC3455-1
26
3455fc
TYPICAL APPLICATIONS
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 suffi cient 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 amplifi er 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 briefl y to provide the higher load currents.
Figure 17. LTC3455/LTC3455-1 Application with 3.3V Output Current Increased to 1.2A
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.
Figure 18. Load Current Step (0.5A to 1.2A) for 3.3V Output
500μs/DIV
V
OUT2
(3.3V)
100mV/DIV
AC-COUPLED
M1 GATE
2V/DIV
I
OUT2
0.5A/DIV
0.5A TO 2A STEP
3455 F18
11
10
13
14
1Ω
C7
4.7μF
C3
1μF
C5
10μF
D1
C4
4.7μF
WALL 5V
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
3.3V, HS
8
6
5
9
1Ω
C6
4.7μF
USB 5V
4
2
3
16
12
249k
L2, 4.7μH
2.49k
80.6k
100pF
C2
2x10μF
3.3V
1.2A
M1
FDN304P
OR
Si2305DS
1
7
100k
L1, 4.7μH
80.6k
10pF
C1
10μF
1.8V
0.4A
24
23
20
21
19
22
15
C8, 0.1μF
1k
2k
V
MAX
1M
2.49k
1M
1.8V
25
3455 F17
μC
USB
CONTROLLER
1.24k
3.32k
ON/OFF
WALLFB
HSI
17
AO
HSO
V
MAX
USB
SUSPEND
USBHP
SW2
AI
18
FB2
SW1
FB1
TIMER
PROG
V
BAT
ON
GND
CHRG
PBSTAT
RST
MODE
LTC3455/LTC3455-1
ON2
PWRON
HSON
SINGLE
CELL Li-ION
3.3V TO 4.2V
+
5.6V