ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 25

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

Standalone USB Power Supply
with Temporary Backup Power
Although designed primarily for Li-Ion powered portable
applications, the LTC3455 is also a good choice for
systems that are always powered by a USB supply or wall
adapter. The battery charger can then be used to charge up
a large capacitor or backup battery, which briefly provides
power to the system after the external power has been
removed. This gives the microcontroller enough time to
follow proper shutdown procedures when the main power
source is abruptly removed. Figure 14 shows a standalone
power supply for USB high power applications (500mA
maximum USB current) using the LTC3455. The total
system power should be kept below 1.8W to ensure clean
operation even under worst-case USB conditions. With
the resistor values shown, the low-battery indicator (AI
and AO pins) triggers when the V
4V, notifying the microcontroller of an impending dropout
condition. The 1MΩ resistor connected between the AI
TYPICAL APPLICATIO S
USB 5V
MAX
4.7µF
U
C6
1Ω
pin voltage drops to
Figure 16. Standalone USB Power Supply with
DROPOUT
5.6V
V
1.8V
MAX
82.5k
CONTROLLER
20k
4700µF
USB
10µF
Temporary Backup Power
C5
C4
10k
V
MAX
2.49k
1M
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%
USB
SUSPEND
USBHP
V
CHRG
WALLFB
TIMER
PROG
V
AO
AI
MAX
BAT
LTC3455
and AO pins provides 150mV of hysteresis (the dropout
indicator stays low until the V
4.15V). A 4700µF 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 10µF 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 4700µF capacitor is always
fully charged to the available USB voltage.
PWRON
PBSTAT
MODE
HSON
HSO
SW2
SW1
GND
ON2
RST
HSI
FB2
FB1
3455 F16
ON
21
15
19
22
20
23
24
14
13
12
18
7
1
25
L2, 4.7µH
L1, 4.7µH
10pF
10pF
1M
1M
ON/OFF
MAX
µC
C3
1µF
3.3V, HS
249k
80.6k
100k
80.6k
pin (using a very large capacitance on
1.8V
C2
10µF
C1
10µF
MAX
3.3V
0.4A
1.8V
0.2A
, the battery charger operates
MAX
pin rises back above
LTC3455
BAT
pin has
25
3455fb
BAT