ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 26

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

LTC3455
Increasing 3.3V Output Current to 1.2A
With an internal current limit of 900mA, Switcher 2 typi-
cally 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. 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
TYPICAL APPLICATIO S
WALL 5V
26
USB 5V
4.7µF
4.7µF
C6
C7
1Ω
3.3V TO 4.2V
1Ω
CELL Li-ION
Figure 17. LTC3455 Application with 3.3V Output Current
SINGLE
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
5.6V
3.32k
1.24k
+
D1
10µF
CONTROLLER
C5
C8, 0.1µF
USB
4.7µF
C4
1k
2.49k
Increased to 1.2A
10
11
8
6
5
4
3
2
9
USB
SUSPEND
USBHP
V
CHRG
WALLFB
TIMER
PROG
V
MAX
BAT
LTC3455
PWRON
PBSTAT
U
MODE
HSON
SW2
SW1
HSO
GND
ON2
RST
HSI
FB2
FB1
ON
3455 F17
AO
AI
21
15
19
22
20
23
24
14
17
13
12
16
18
7
1
25
L2, 4.7µH
L1, 4.7µH
100pF
10pF
1M
C3
1µF
3.3V, HS
2k
1M
ON/OFF
µC
249k
2.49k
80.6k
100k
80.6k
1.8V
M1
FDN304P
OR
Si2305DS
C2
2x10µF
C1
10µF
V
3.3V
1.2A
1.8V
0.4A
MAX
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.
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
0.5A TO 1.2A STEP
AC COUPLED
V
100mV/DIV
OUT2
M1 GATE
0.5A/DIV
2V/DIV
(3.3V)
I
OUT2
500µs/DIV
3455 F18
3455fb