LTC3455EUF#TRPBF Linear Technology, LTC3455EUF#TRPBF Datasheet - Page 26

• Manufacturer Part Number: LTC3455EUF#TRPBF
• Description: IC CONV DC/DC DUAL USBBATT 24QFN
• Manufacturer: Linear Technology

Specifications of LTC3455EUF#TRPBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 3 V ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -

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