LTC3441 Linear Technology, LTC3441 Datasheet - Page 22

• Manufacturer Part Number: LTC3441
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

LTC3455
USB 5V
and AO pins provides 150mV of hysteresis (the dropout
indicator stays low until the V
4.15V). A 4700 F backup capacitor connected to the V
pin briefly provides power to the system after the USB
supply has been removed, and also helps support tran-
sient loads that slightly exceed the USB current limit.
Connecting this large capacitance to the V
several advantages. It provides a large energy reservoir
that is isolated from both the USB pin (the USB specifica-
tion limits capacitance on the USB supply pin to 10 F or
less) and the V
this pin will delay the system turn-on), and it prevents
large inrush currents by using the battery charger to
slowly charge this capacitor (normally using such a large
capacitor would result in very large inrush currents). With
the TIMER pin tied to V
in constant-current mode (the voltage-loop and timer
function are disabled), so the 4700 F capacitor is always
fully charged to the available USB voltage.
TYPICAL APPLICATIO S
22
4.7 F
C6
1
DROPOUT
Figure 14. Standalone USB Power Supply with
V
1.8V
MAX
82.5k
CONTROLLER
20k
4700 F
USB
10 F
C5
C4
10k
V
MAX
2.49k
MAX
1M
Temporary Backup Power
10
11
17
16
8
6
5
4
3
2
9
C1, C2, C3, C5, C6: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
ALL RESISTORS 1%
pin (using a very large capacitance on
USB
SUSPEND
USBHP
V
CHRG
WALLFB
TIMER
PROG
V
AO
AI
MAX
BAT
MAX
LTC3455
, the battery charger operates
PWRON
PBSTAT
MODE
HSON
SW2
SW1
HSO
GND
ON2
RST
3455 TA02
HSI
FB2
FB1
ON
MAX
U
21
15
19
22
20
23
24
14
13
12
18
7
1
25
pin rises back above
L2, 4.7 H
L1, 4.7 H
10pF
10pF
1M
1M
ON/OFF
BAT
C3
1 F
3.3V, HS
C
249k
80.6k
100k
80.6k
1.8V
pin has
C2
10 F
C1
10 F
3.3V
0.4A
1.8V
0.2A
BAT
WALL 5V
USB 5V
4.7 F
1 F
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 15 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.
3.3V TO 4.2V
C7
C6
CELL Li-ION
1
1
Figure 15. 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%
CONTROLLER
3.32k
1.24k
+
D1
USB
10 F
C5
C8, 0.1 F
4.7 F
C4
1k
2.49k
10
11
Increased to 1.2A
8
6
5
4
3
2
9
USB
SUSPEND
USBHP
V
CHRG
WALLFB
TIMER
PROG
V
MAX
BAT
LTC3455
PWRON
PBSTAT
MODE
HSON
SW2
SW1
HSO
GND
ON2
RST
HSI
FB2
FB1
3455 F15
ON
AO
AI
21
15
19
22
20
23
24
14
17
13
12
16
18
7
1
25
L2, 4.7 H
L1, 4.7 H
100pF
10pF
1M
C3
1 F
3.3V, HS
1k
1M
ON/OFF
249k
2.49k
80.6k
100k
80.6k
C
1.8V
M1
FDN304P
OR
Si2305DS
C2
2x10 F
3.3V
1.2A
1.8V
0.4A
C1
10 F
V
3455f
MAX