LTC3455 Datasheet, PDF (22/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
TYPICAL APPLICATIO S
USB 5V
1Ω
C6
4.7µF
USB
CONTROLLER
8
USB
6
SUSPEND
5
USBHP
C5
10µF
10
V_MAX
21
MODE
15
HSON
19
ON2
22
PWRON
20
RST
23
PBSTAT
LTC3455
1M 1M
µC
1.8V
4
CHRG
11 _WALLFB
3
V_MAX
2
2.49k
9
C4
4700µF
1.8V
DROPOUT
10k
17
V_MAX
82.5k
1M
16
20k
TIMER
PROG
V_BAT
AO
AI
24
ON
14
HSO
ON/OFF
3.3V, HS
C3
1µF
13
HSI
12 ^{L2, 4.7µH}
SW2
3.3V
0.4A
10pF
18
FB2
249k
C2
10µF
80.6k
7 ^{L1, 4.7µH}
SW1
10pF
1
FB1
25
GND
100k
80.6k
1.8V
0.2A
C1
10µF
3455 TA02
C1, C2, C3, C5, C6: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
ALL RESISTORS 1%
8
USB 5V
6
1Ω
C6
4.7µF
USB
CONTROLLER
5
WALL 5V
1Ω
C7
1µF
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
V_MAX
23
PBSTAT
LTC3455
CHRG
ON
WALLFB
HSO
TIMER
PROG
AO
V_BAT
HSI
SW2
AI
FB2
1M 1M
1.8V
24
ON/OFF
14
3.3V, HS
C3
1µF
V_MAX
1k
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 F15
100k
80.6k
1.8V
0.4A
C1
10µF
Figure 14. Standalone USB Power Supply with
Temporary Backup Power
Figure 15. LTC3455 Application with 3.3V Output Current
Increased to 1.2A
and AO pins provides 150mV of hysteresis (the dropout
indicator stays low until the V_MAX pin rises back above
4.15V). A 4700µF backup capacitor connected to the V_BAT
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_BAT pin has
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 10µF or
less) and the V_MAX pin (using a very large capacitance on
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_MAX, the battery charger operates
in constant-current mode (the voltage-loop and timer
function are disabled), so the 4700µF capacitor is always
fully charged to the available USB voltage.
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.
3455f
22