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
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 VMAX pin voltage drops to
4V, notifying the microcontroller of an impending dropout
condition. The 1MΩ resistor connected between the AI
and AO pins provides 150mV of hysteresis (the dropout
indicator stays low until the VMAX pin rises back above
4.15V). A 4700µF backup capacitor connected to the VBAT
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 VBAT 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 VMAX 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 VMAX, 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.
USB 5V
1Ω
C6
4.7µF
USB
CONTROLLER
8
USB
6
SUSPEND
5
USBHP
C5
10µF
10
VMAX
21
MODE
15
HSON
19
ON2
22
PWRON
20
RST
23
PBSTAT
LTC3455
1M 1M
µC
1.8V
4 CHRG
11 WALLFB
3
VMAX
2
2.49k
9
C4
4700µF
1.8V
DROPOUT
10k
17
VMAX
82.5k
1M
16
20k
TIMER
PROG
VBAT
AO
AI
24
ON
ON/OFF
14
HSO
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 F16
C1, C2, C3, C5, C6: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
ALL RESISTORS 1%
Figure 16. Standalone USB Power Supply with
Temporary Backup Power
3455fa
25
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