DN108 - 250kHz, 1mA I Q Constant Frequency Switcher Tames Portable Systems Power

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250kHz, 1mA IQ Constant Frequency Switcher Tames
Portable Systems Power – Design Note 108
Bob Essaff
DC-to-DC power conversion remains one of the toughest tasks for portable system designers. Dealing with
various battery technologies and output voltage requirements dictate the need for creative circuit solutions. The
two circuits discussed are tailored for operation from
a single lithium-ion (Li-Ion) cell. These new batteries
are finding widespread use due to their high energy
storage capabilities. The first circuit has a 3.3V output
and the second circuit has both 5V and –5V outputs
for applications requiring dual supplies.
At the heart of each circuit is the LT®1373 current mode
switching regulator. Guaranteed to operate down to
2.7V, this part allows the full energy storage capacity
of a single Li-Ion battery to be used. The LT1373 draws
only 1mA of quiescent current for high efficiency at
light loads and has a low resistance 1.5A switch for
good efficiency at higher loads. Switching at 250kHz
saves space by reducing the size of the magnetics,
and the fixed-frequency switching also reduces the
noise spectrum generated. To avoid sensitive system
frequencies the part can be externally synchronized
to a specific frequency from 300kHz to 360kHz. The
LT1373 can also be shut down where it draws only
12μA supply current.
3.3V SEPIC Converter
Generating a 3.3V output from a single Li-Ion cell is
not straight forward because at full charge the battery
voltage is above the output voltage and when discharged,
OFF
+
SINGLE
Li-Ion
CELL
+
5
VIN
8
ON 4 S/S
VSW
LT1373
2
FB
GND VC
C1
100μF
10V
6, 7
L1A
33μH
the battery voltage is below the output voltage. A
conventional buck or boost regulator topology will not
work. The circuit in Figure 1 uses the SEPIC (singleended primary inductance converter) topology which
allows the input voltage to be higher or lower than the
output voltage. The circuit’s two inductors, L1A and
L1B, are actually two identical windings on the same
inductor core, though two individual inductors can
be used. The topology is essentially identical to a 1:1
transformer-flyback circuit except for the addition of
capacitor C2 which forces identical AC voltages across
both windings. This capacitor performs three tasks.
First, it eliminates the power loss and spikes created
by flyback-converter leakage inductance. Secondly, it
forces the input current to be a triangular waveform
riding on top of a DC component instead of forming a
large amplitude square wave. Finally, it eliminates the
voltage spike across the output diode when the switch
turns on. Another feature of the SEPIC topology is that,
unlike a typical boost converter, there is no DC path
from the input to the output. This means that when the
LT1373 is shut down, the load is completely disconnected from the input power source. Figure 2 shows
that the 3.3V SEPIC converter maintains reasonable
efficiency over two decades of output load current even
though 3.3V circuits typically have low efficiency due
to catch diode losses.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
D1
MBRS130LT3
•
C2
2.2μF
•
L1B
33μH
VOUT
3.3V
R2
41.2k
1% +
1
R1
5k
C4
0.01μF
R3
24.9k
1%
C3
100μF
10V
DN108 • F01
C1, C3: AVX TPSD 107M010R0100
C2: TOKIN 1E225ZY5U-C203-F
L1: COILTRONICS CTX33-2, SINGLE
INDUCTOR WITH TWO WINDINGS
Figure 1. Single Li-Ion Cell to 3.3V SEPIC Converter
07/95/108_conv
–5V supply which is only quasi-regulated due to diode
drop and switch saturation losses. Figure 4 shows the
regulation of the negative output for various positive
output load currents. As shown in Figure 5, the dual
output converter has high efficiency over two decades
of load current.
90
VIN = 4V
80
EFFICIENCY (%)
VIN = 3V
70
60
For more information, please consult the LT1373 data
sheet. For parts similar to the LT1373 with higher
switching frequencies (500kHz and 1MHz), consult the
LT1372/LT1377 data sheet.
50
40
5
10
50
100
OUTPUT LOAD CURRENT (mA)
500
– 5.4
DN108 • F02
+
C2
100μF
L1
10μH
5
VIN
4
S/S
VSW
8
D1
LT1373
+
SINGLE
Li-Ion
CELL
1
VC
FB
2
+
R3
24.9k
1%
R1
GND
2k
6, 7
C1
0.01μF
L1: SUMIDA CD54-100
C2 TO C5: AVX TPSD 107M010
D1 TO D3: MBRS130LT3
5V
R2
75k
1%
– 5.2
– 5.0
– 4.8
– 4.6
5V LOAD = 50mA
– 4.4
– 4.2
200mA
– 4.0
400mA
– 3.8
0
10 20 30 40 50 60 70 80 90 100
NEGATIVE LOAD CURRENT (mA)
DN108 • F04
Figure 4. – 5V Regulation
100
VIN = 4V
90
EFFICIENCY (%)
Dual Output Converter
Many portable systems still require a negative bias
voltage to operate interface or other circuitry, where the
voltage accuracy is not critical. Using a single inductor,
the circuit in Figure 3 generates a regulated 5V output
and a quasi-regulated –5V output for such applications.
The circuit first converts a single Li-Ion cell input voltage
to a well-regulated 5V output. It then takes advantage
of the switching waveform on the VSW pin to generate
the –5V output in a charge pump fashion. The voltage
on the VSW pin is 5V plus D1’s forward voltage when
VSW is high. At this time, C3 charges to the VSW voltage
minus D2’s forward voltage or about 5V. When the VSW
pin goes low, the minus side of C3 goes to –5V which
turns on D3 and charges C5 to –5V. This generates a
NEGATIVE OUTPUT VOLTAGE (V)
VIN = 3V
Figure 2. 3.3V Efficiency
VIN = 3V
80
70
60
– 5V CURRENT = 20%
OF 5V CURRENT
C4
100μF
50
5
10
50
100
POSITIVE LOAD CURRENT (mA)
500
DN108 • F05
+
C3
100μF
D2
+
D3
C5
100μF
R4
24k
Figure 5. Dual Output Efficiency
– 5V
DN108 • F03
Figure 3. Single Li-Ion to ± 5V
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn108f_conv LT/GP 0795 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1995
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