Single Boost Converter
Builds Dual Polarity Supply
By Yogesh Sharma, Applications Engineer, Standard Power
Products Group, Analog Devices, San Jose, Calif.
Adding charge pump and LDO components to
a boost converter circuit creates a cost-effective power supply for LCDs and other portable
applications.
M
any portable devices use LCD displays. These
size of the inductor changes with the frequency at which the
displays need large positive and negative
PWM controller is switching, which is set by the logic pin
power supplies, but most mobile devices
RT. Also, the diode DOUT+ should be chosen such that it can
have power rails at 5 V, 3.3 V or even lower.
conduct the average output current.
Thus, the challenge is not only to boost the
A boost converter can also be used to generate a negative
input power supply, typically between 3 V and 5 V, to around
voltage rail by using a charge-pump configuration (Fig. 2). In
+12 V, but also to generate the negative voltage rail, usuthis circuit, when the switch turns off, the inductor dumps
ally –6 V. Typically, the +12-V rail is generated by a boost
current into charge-pump capacitor C1 and diode D1 to
converter, such as Analog Devices’ ADP1611 (Fig. 1).
ground. When the switch turns on, the voltage on capacitor
The ADP1611 consists of a power switch, a PWM controlC1 forward biases diode D2. The charge held in C1 transfers
ler, a reference voltage source and an error amplifier. When
through diode D2 to the output capacitor (COUT–), which
the switch (internal to the ADP1611) turns on, the SW node
draws current out of the load. The on and off times of the
is grounded, leading to current buildup in the inductor.
switch can be modulated to regulate the negative output
When the switch turns off, the stored inductor current flows
voltage rail.
through the diode DOUT+ to the output capacitor and load,
In Fig. 1, when the divided output voltage presented at FB
generating an output voltage greater
�����������
than the input voltage. The error
amplifier in the ADP1611 measures
the output voltage through a voltage
���
divider and creates an error signal
��
�����
������
that allows the PWM controller to
�����
�����
drive the switch with the correct on
�����
and off times to maintain the desired
��� �� ��
output voltage and load current.
��
� �����
� ����
�������
The output voltage is set by the
�������
�����
ratio of the resistor divider formed
������
���
�������������
by resistors R1 and R2 according to
����������
the following equations:
�������
 R1 
VOUT+ = 1.223 V  1+ 
 R2 
R 2 = 10 kΩ
V

R1=R2  OUT+ − 1 .
 1.23 V 
The value of the input and output
capacitor are fixed at 10 µF, while the
Power Electronics Technology September 2006
���
�������
��
�����
��
���
Fig. 1. A boost converter being used to generate an output voltage of +12 V from an input
voltage of 3.6 V.
42
www.powerelectronics.com
BOOST CONVERTER
increases, the control system decreases
the inductor current. This maintains
the output voltage at the regulation
voltage. In the Fig. 2 circuit, however,
the output voltage may not be divided
down to control the regulator because
the output voltage is negative, but the
control circuit regulates the voltage at
the FB pin to +1.23 V.
Also, increasing inductor current
results in a more negative output voltage, so feeding a divided version of the
output voltage would cause positive
feedback, and thus an unstable regulation circuit. Instead, an operational
amplifier used in an inverting configuration provides level shifting and
polarity inversion, thus presenting a
ratiometric version of the output voltage at FB and allowing proper control
of the output voltage.
A general-purpose op amp—such
as the AD8541 shown in Fig. 2—is
sufficient for this application. This amplifier should have a gain bandwidth
product above 100 kHz and be able to
source a few hundred microamperes
of current. Note also that the op amp
should support positive single-supply
�����������
��
���
�����
� ����
��
������
��� ��
��
�������
� �����
��
������
��������
�������������
�����������
���������
���
����������
�������
���
�������
��
����
��
���
��
������
��
�����
����
������ �
��
�����������
���
������ ���
��
�������������������
Fig. 2. This inverting boost converter being used to generate an output voltage of –12 V from an
input voltage of 3.6 V.
���������������������������
�����������
��
��
������
��
��
�����������
����������
���
�����������
�����������
���
��
���
���
�����
�����
��� ��
��
� ����
�����
�������
��
��
������
�����
�����
��
�������
�����
���
������
������
������������
�����������
� �����
������
�������������
���
����������
�������
���
�������
��
�
�������
��
�������
��
�����
��
���
Fig. 3. Using the same boost converter to generate both the positive (+12 V) and negative (–6 V) voltage rails.
Power Electronics Technology September 2006
44
www.powerelectronics.com
BOOST CONVERTER
operation with a common-mode input
voltage range that includes ground.
The ratio of the feedback resistors sets
the ratio of the absolute value of the
output voltage to the voltage reference
of the boost converter. Thus in Fig. 2,
to regulate an output voltage of –12 V,
the resistor values are R1 = 100 k
and R2 = 10 k. The charge-pump
capacitor C1 (typically 1 µF) should
be large enough to hold the charge
that needs to be drawn from the load
during a single switching cycle.
While the above solutions show
that a boost converter can be used to
generate either a large positive or a
large negative voltage rail, it is more
cost-effective to use the same boost
converter to generate both positive and
negative voltage rails. The challenge in
doing this is that the boost converter
can only regulate the output voltage
Achieve higher performance from transformers with
HYPER-X
MAGNETIC TECHNOLOGY ™
HYPER-XMT™ at work
720 watts
BRIDGE TRANSFORMER
Surprisingly
powerful
99.3% efficiency*
Computer server applications
360 watts
RESONANT TRANSFORMER
Designed for high efficiency Plasma TV
power supplies using ON Semiconductor
MC34067 resonant mode controller
Enables Ultra-high
Power Density
Components—
800 watts
BOOST INDUCTOR
99.3% efficiency*
Industrial power applications
the smallest, lightest,
most efficient
magnetics possible.
The magnetics:
surprisingly cool.
The technology:
100 watts
FORWARD TRANSFORMER
99.2% efficiency*
Portable power applications
RED HOT
*Contact us. Let’s talk about the ways we can
improve the efficiency of your power supply.
toll-free: 1-888-876-6424 | www.tabtronics.com
Power Electronics Technology September 2006
46
of one rail. Thus, the boost converter
regulates one of the voltage rails (positive or negative) while the other rail is
post-regulated.
The voltage rail that is being regulated by the boost converter must have
a higher magnitude than the other rail.
This is the positive rail in most LCD
applications. A simple circuit is shown
in Fig. 3 where the boost converter
is regulating the positive voltage rail
(+12 V) while the negative rail (–6 V)
is generated by a linear regulator, such
as Analog Devices’ ADP3331, which
post-regulates the unregulated chargepump output.
When the ADP1611 switch turns
off, L1 dumps current through DOUT+
into COUT+, which is regulated to +12
V. The potential on the SW terminal
of the ADP1611 will then be one
diode drop above +12 V. Unlike the
conventional method of post-regulating a switcher with an LDO, the LDO
in this circuit is not directly regulating the load. Rather, it is acting as a
virtual programmable Zener placed
between its IN and OUT terminals that
controls the charging of C1 to ensure
COUT– maintains a voltage of 6 V.
Thus, if the voltage across COUT– exceeds 6 V, the pass element resistance
of the LDO will increase, reducing the
charge rate on C1 when the ADP1611
switch is open. This will reduce the
amount of replenishing charge transferred from C1 to COUT– through D2
when the switch closes. Conversely,
when the voltage across COUT– is below
6 V, the pass-element resistance decreases, and the replenishing charge to
COUT– will increase. Because the OUT
terminal of the LDO is tied to ground,
VOUT– is regulated to –6 V.
This topology offers a simple, costeffective solution to generate large positive and negative voltages using just a
single boost converter, a conventional,
low-cost positive supply LDO and a few
diodes. It does have a few limitations:
the magnitude of the positive rail must
be greater than that of the negative rail,
and the boost converter must be capable of supplying the total maximum
load current of both rails.
PETech
www.powerelectronics.com