LTC3455(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC3455
(Rev.:RevA)
(Rev.:RevA)
Linear Technology
LTC3455 Datasheet PDF : 28 Pages
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LTC3455
TYPICAL APPLICATIO S
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
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.
8
USB 5V
6
1Ω
C6
4.7µF
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
20
RST
VMAX
23
PBSTAT
LTC3455
CHRG
ON
WALLFB
HSO
TIMER
PROG
AO
VBAT
HSI
SW2
AI
FB2
1M 1M
1.8V
24
ON/OFF
14
3.3V, HS
C3
1µF
VMAX
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
100k
80.6k
1.8V
0.4A
C1
10µF
VOUT2 (3.3V)
100mV/DIV
AC COUPLED
IOUT2
0.5A/DIV
0.5A TO 1.2A STEP
M1 GATE
2V/DIV
500µs/DIV
3455 F18
Figure 18. Load Current Step (0.5A to 1.2A) for 3.3V Output
Figure 17. LTC3455 Application with 3.3V Output Current
Increased to 1.2A
3455fa
26
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