DN189 - Step-Up/Step-Down DC/DC Conversion Without Inductors

Step-Up/Step-Down DC/DC Conversion without Inductors
Design Note 189
Sam Nork
A unique architecture allows the parts to accommodate
a wide input voltage range (2.0V to 10V) and adjust the
operating mode as needed to maintain regulation. As
a result, the parts can be used with a wide variety of
battery configurations and/or adapter voltages (Figure 2). Low power consumption (IQ = 60μA typical)
and low external parts count make the LTC1514 and
LTC1515 well suited for space-conscious, low power
applications, such as cellular phones, PDAs and portable instruments. The parts come in adjustable and
fixed output voltages and include additional features
such as power-on reset capability (LTC1515 family) and
an uncommitted comparator that is kept alive during
shutdown (LTC1514 family).
Regulator Operation
The parts use a common internal switch network to
implement both step-up and step-down DC/DC conversion. The action of the switch network is controlled by
internal circuitry that senses the voltage differential
between VIN and VOUT. When the input voltage is lower
than the output voltage, the switch network operates as
a step-up voltage doubler with a free-running frequency
set by the internal oscillator (650kHz typ). When the input
voltage is greater than the output, the switch network
operates as a step-down gated switch. Regulation is
achieved by comparing the divided output voltage to
the internal reference voltage. When the divided output
drops below the reference voltage, the switch network
is enabled to boost the output back into regulation. The
net result is a stable, tightly regulated output supply
that can tolerate widely varying input voltages and load
transients (Figures 3 and 4).
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5.2
IOUT = 10mA
5/3 = 3V
RESET
ON OFF
LTC1515-3.3/5
1
2
μC
5V 3.3V
3
4
SHDN VOUT
POR
VIN
5/3
C1+
GND
C1–
100k
VOUT = 3.3V ±4%
OR 5V ±4%
IOUT = 0 TO 50mA
8
7
6
5
OUTPUT VOLTAGE (V)
Introduction
Many applications require a regulated supply from an
input source that may be greater than or less than the
desired output voltage. Such applications place unique
constraints on the DC/DC converter and, as a general
rule, add complexity and cost to the power supply. A
typical example is generating 5V from a 4-cell NiCd
battery. When the batteries are fully charged, the
input voltage is around 6V. When the batteries are
near end of life, the input voltage may be as low as
3.6V. Maintaining a regulated 5V output for the life
of the batteries typically requires an inductor-based
DC/DC converter (for example, a SEPIC converter) or
a complex, hybrid step-up/step-down solution. The
LTC ®1514/LTC1515 family of switched capacitor DC/DC
converters handles this task using only three external
capacitors (Figure 1).
5.1
5.0
4.9
+
10μF
0.22μF
VIN
4 CELLS
+
10μF
DN189 F01
4.8
2
3
4
5
8
6
7
INPUT VOLTAGE (V)
9
10
DN188 F02
Figure 1. Programmable 5V/3V Power Supply
with Power-On Reset
09/99/189_conv
Figure 2. LTC1515-X 5V
Output Voltage vs Input Voltage
VOUT
AC COUPLED
100mV/DIV
VOUT
AC COUPLED
100mV/DIV
50mA
IOUT
50mA/DIV
IOUT
50mA/DIV
0mA
VIN = 8V
VOUT = 5V
DN189 F03
COUT = 10μF
TA = 25°C
VIN = 3.3V
VOUT = 5V
DN189 F04
COUT = 10μF
TA = 25°C
Figure 3. LTC1515-X Step-Down Mode
5V Load Transient Response
Figure 4. LTC1515-X Step-Up Mode
5V Load Transient Response
Dual Output Supply from a 2.7V to 10V Input
The circuit shown in Figure 5 uses the low-battery
comparator to produce an auxiliary 3.3V regulated
output from the VOUT of the LTC1514-5. A feedback
voltage divider formed by R2 and R3 connected to the
comparator input (LBI) establishes the output voltage.
The output of the comparator (LBO) enables the current
source formed by Q1, Q2, R1 and R4. When the LBO
pin is low, Q1 is turned on, allowing current to charge
output capacitor C4. Local feedback formed by R4, Q1
and Q2 creates a constant current source from the 5V
output to C4. Peak charging current is set by R4 and
the VBE of Q2, which also provides current limiting in
the case of an output short to ground. With the values
shown in Figure 5, the auxiliary regulator can deliver
up to 50mA before reaching its current limit. However,
the combined output current from the 5V and 3.3V
supplies may not exceed 50mA. Since the regulator
implements a hysteretic feedback loop in place of the
traditional linear feedback loop, no compensation is
needed for loop stability. Furthermore, the high gain
of the comparator provides excellent load regulation
and transient response.
Conclusion
With low operating current, minimal external parts
count and robust protection features, the LTC1514 and
LTC1515 offer a simple and cost-effective solution to
low power step-up/step-down DC/DC conversion. The
shutdown, POR and low-battery-detect features provide
additional functionality. The ease of use and versatility
of these parts make them ideal for low power DC/DC
conversion applications.
ON OFF
R4
10Ω
Q1
TPO610T
VOUT
3.3V
±4%
+
C4
10μF
R3
750k
1%
R2
402k
1%
Q2
MMBT3906LT1
220k
LTC1514-5
1
R1
47k
2.2nF
2
3
4
SHDN VOUT
8
7
LBO
VIN
LBI
C1+
6
GND
C1–
5
+
C1
0.22μF
C3
22μF
C2
10μF
VOUT
5V ±4%
VIN
2.7V TO 10V
+
DN189 F05
Figure 5. Low Power Dual Output Supply (Maximum Combined IOUT = 50mA)
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn189f_conv LT/TP 0998 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1998
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