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Multiple Output DC/DC Converter Runs Off 2-Cell AA Batteries,
USB or AC Wall Adapter – Design Note 1006
Gurjit Ghandi and Minjong Kim
The 3.3V and core supplies are generated by switching
regulators operating at 1MHz constant frequency. High
switching frequency allows the use of tiny, low profile
inductors and capacitors. All switchers are current mode
PWM type with internal compensation, reducing pin and
part counts.
Introduction
The LTC®3456 is a low profile, total power management
solution optimized for the confined spaces and diverse
power requirements of mobile devices, including MP3
players, portable GPS units, digital cameras and handheld
computers. The LTC3456 seamlessly manages power
flow between an AC adapter, USB cable and a 2-cell AA
battery while complying with USB power standards and
fits in a 4mm × 4mm 24-pin QFN package.
Complete Power Solution for Handhelds from
Two AA Cells
The circuit shown in Figure 1 generates five output
voltages from one of three power sources: AC adapter,
USB or 2-cell alkaline battery. The first output is the Core
The device generates two separate power rails: a 3.3V
(fixed) main supply and a 1.8V (adjustable) core supply.
In addition, the LTC3456 contains a USB power manager,
a Hot SwapTM output, a low-battery indicator and an
always-alive VMAX output.
, LTC, LT and Burst Mode are registered trademarks of Linear Technology
Corporation. Hot Swap is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
3.3V (FIXED)
2 AA
CELLS
+
C1
4.7µF
L2
4.7µH
SW2_BST
SW2_BK
100mA/500mA
ON OFF
VINT
VMAIN
VBATT
USB
CONTROLLER
C10
22µF
L3
10µH
USBHP
C9
1µF
MAIN OUTPUT
3.3V
150mA
100k
Q1 LINEAR LDO REGULATOR
A0
LCD LOGIC BIAS
2.8V
C8
5mA
2.2µF
SUSPEND
49.9k
VBUS
USB POWER
(4.35V TO 5.5V)
C2
4.7µF
AIN
USB
20k
1Ω
LED ON WITH
EXTERNAL POWER
HSO
C7
1µF
L1
10µH
1k
EXT_PWR
C4
10µF
D1
AC
ADAPTER
(5V ±10%)
LTC3456
SW1
VEXT
100k
220pF
FLASH MEMORY CARD
3.3V (SHORT-CIRCUIT PROTECTED)
50mA
CORE OUTPUT
1.8V
200mA
C6
10µF
FB1
80.6k
11.3k
WALLFB
C3
4.32k
4.7µF
PWRKEY
VMAX
PBSTAT RESET MODE PWRON PGND AGND
1Ω
C5
1µF
VMAX (HIGHEST OF VBATT, VINT, VEXT AND USB)
1mA
(TO REAL-TIME CLOCK)
ON/OFF
DN1006 F01
MICROCONTROLLER
L1, L3: MURATA LQH32CN100K53
C1, C6 TO C10: X5R OR X7R, 4V
L2: MURATA LQH32CN4R7M53
C2 TO C5: X5R OR X7R, 6.3V
D1: ON SEMICONDUCTOR MBRM120E Q1: PHILIPS MMBT3906
Figure 1. Complete Power Supply for Handheld Devices. Includes Two Switching Channels and One Linear Regulator
09/05/1006
1.8V, 200mA supply for the system processor. This is an
adjustable output set by feedback resistors connected to
the FB1 pin. This output uses internal synchronous power
switches, improving efficiency and eliminating the need
for an external Schottky diode. The second output is the
3.3V, 150mA Main output for peripheral circuitry. The
third output is a second 3.3V supply (at 50mA) at the Hot
Swap output (HSO) for hot swappable memory cards.
The fourth output, which is generated from the Main
output using an internal gain block, provides 2.8V at 5mA
which is suitable for biasing LCD drivers. The fifth output
is VMAX which is a direct unregulated output of whichever
is the highest available supply voltage: VBATT, VINT, VEXT
or USB voltage. The VMAX output can be used to supply up
to 1mA to a critical block like a real-time clock which needs
to stay alive at all times.
The LTC3456 provides power supply sequencing for all
outputs. After start-up, VINT powers up, followed by the
Core, Main and Hot Swap outputs. During shutdown, both
VMAIN and Hot Swap outputs are discharged to ground via
internal pull-down resistors. VCORE is also discharged in
this manner, preventing the microprocessor from entering into a spurious operating mode.
Figure 2 shows the efficiency of the core converter when
powered by a 2-cell alkaline. Efficiency reaches 93% at the
higher load currents. Burst Mode® operation improves
efficiency at light loads.
Figures 3 and 4 show the stable load step response of the
Core and Main outputs—±3% deviation from nominal—
for a 20mA to 150mA load transient on the Core output
and a 20mA to 100mA transient on the Main output.
Conclusion
With its low profile and seamless transition between input
power sources, the LTC3456 simplifies and shrinks power
supply designs for portable devices such as personal GPS
navigators, MP3 players, digital cameras and handheld
computers.
VCORE
100mV/DIV
(AC COUPLED)
ISW1
200mA/DIV
VMAIN
200mV/DIV
(AC COUPLED)
ISW2(BST)
200mA/DIV
100
100µs/DIV
DN1006 F03
90
Figure 3. Load Transient (BATT = 2.4V)
EFFICIENCY (%)
80
70
60
50
40
30
20
VBATT = 2.4V
VCORE = 1.8V
10
0
PWM MODE
Burst Mode
OPERATION
10
100
LOAD CURRENT (mA)
1
1000
DN1006 F02
Figure 2. Core Converter Efficiency of the LTC3456
When Powered by a 2-Cell Alkaline
VCORE
100mV/DIV
(AC COUPLED)
ISW1
200mA/DIV
VMAIN
200mV/DIV
(AC COUPLED)
ISW2
200mA/DIV
200µs/DIV
DN1006 F04
Figure 4. Load Transient (AC Wall/USB = 5V)
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900, Ext. 2759
dn1006f 0905 • PRINTED IN THE USA
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