Dec 2002 Replace Two ICs with a Combination High Efficiency Buck Controller Plus Low Noise LDO

DESIGN IDEAS
Replace Two ICs with a Combination
High Efficiency Buck Controller Plus
Low Noise LDO
by Mark Vitunic
VIN
5V
DESIGN IDEAS
Replace Two ICs with a Combination
High Efficiency Buck Controller Plus
Low Noise LDO ............................ 34
VOUT1
3.3V
AT
500mA
Mark Vitunic
A Simple Solution to Low Noise,
Isolated Power Conversion .......... 35
L1
15µH
Bootstrapped Power Supply Permits
Single Rail Amplifier Output Swing to
Ground (and Below) ..................... 36
+
34
6
M1
SENSE –
LDO
2
169k
PGATE
VFB2
249k
3
VOUT2
2.5V AT
150mA
C3
2.2µF
16V
LTC3700
D1
80.6k
9
10
Jim Williams
220pF
VFB
PGOOD
ITH/RUN
GND
4
5
10k
C1: TAIYO YUDEN EMK325BJ106MNT
C2: SANYO POSCAP 6TPA47M
C3: MURATA GRM42-6X7R225K016AL
D1: MOTOROLA MBRS130LT3
L1: COILTRONICS UP1B150
M1: SILICONIX Si3443DV
R1: DALE 0.25W
(408) 573-4150
(619) 661-6835
(770) 436-1300
(800) 441-2447
(561) 752-5000
(800) 554-5565
(605) 665-9301
Figure 1. 5V input to dual output: a 3.3V/500mA high
efficiency output and a 2.5V/150mA low noise output
regulator’s input supply, its own independent input supply or the buck
controller’s output. The LDO is protected by both current limit and
thermal shutdown circuits.
The LTC3700 provides ±2.5% output voltage accuracy for both the
buck and LDO. The buck consumes
only 210µA of quiescent current in
normal operation with the LDO con100
VIN = 5V
VOUT = 3.3V
90
EFFICIENCY (%)
Need a second low-noise voltage output alongside your DC/DC converter,
but don’t have the room for another
IC? The LTC3700 offers a simple solution by combining a constant
frequency current mode step-down
DC/DC controller with a 150mA low
dropout (LDO) regulator in a tiny 10pin MSOP.
The buck controller section of the
LTC3700 offers many of the features
expected in a high-performance
switcher: high efficiency (up to 94%),
wide VIN range (2.65V to 9.8V), high
constant frequency operation (550kHz),
and current mode control for excellent AC and DC load and line
regulation. The buck is configured for
Burst Mode® operation, which reduces
switching losses at light load, thereby
enhancing efficiency. In dropout, the
external P-channel MOSFET is turned
on continuously (100% duty cycle),
extending the usable voltage range of
a battery source.
The LDO output is powered by an
internal P-channel MOSFET pass device with an on resistance of
approximately 1.5Ω (with VIN2 = 4.2V).
The LDO has a separate input supply
pin, which offers the versatility of
powering the LDO from the buck
8
1
VIN2
80.6k
C2
47µF
6V
Tom Sheehan
7
VIN
R1
0.1Ω
C1
10µF
16V
80
70
60
50
40
1
100
10
LOAD CURRENT (mA)
1k
Figure 2. Efficiency of the 3.3V
output for the circuit in Figure 1
suming an additional 50µA. In shutdown, a mere 10µA (combined) is
consumed. A common “Power Good”
output monitors both supplies.
5V Input Supply to 3.3V/
500mA High Efficiency
Output and 2.5V/150mA Low
Noise Output
Figure 1 shows a dual regulated output voltage design running off of a
single 5V input supply. The input to
the LDO, VIN2, could connect directly
to VIN, but better efficiency is obtained by running if off the 3.3V buck
output.
Figure 2 shows the buck efficiency
vs load current. Since the LDO’s input supply is connected to the buck
output, input current to the LDO
adds to the load current seen by the
buck. With both outputs running at
maximum current, 500mA (buck) and
150mA (LDO) for 650mA total, the
measured buck efficiency was 91.4%.
Linear Technology Magazine • December 2002
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