990514
15/-9 V 8-Bit Programmable Power Supply Using the S-8330
The power supplies for LCD panels, widely
employed in portable electronic devices, call for the
use of a booster circuit because they normally require
a higher positive voltage than the battery voltage.
The booster circuit must be able to perform the
following functions: i) regulate the boosted positive
voltage value externally to adjust the contrast of the
panel, ii) discharge the positive voltage to 0 V so that
after-images are prevented when the power is shut off.
Furthermore, when the SA (Smart Addressing)
drive system or the MLA (Multiline Addressing) drive
system is adopted, a negative voltage power supply is
also needed that works in interlock with the positive
voltage value regulated externally.
This article will introduce a power supply circuit that
has successfully implemented the functions mentioned
above by using the S-8330, an IC which is suitable for
building up an LCD panel power supply.
Basic operation of the S-8330
The S-8330 is a PWM-controlled boosting type
switching regulator IC that incorporates an electronic
volume (electronic voltage control) for output voltage
adjustment, and a switching transistor.
Figure 1 represents the basic operation circuit of
the S-8330. As the built-in switching transistor (Msw)
is turned on and off, the energy stored in the inductor
(L) is released through the output to generate a high
positive voltage, and the on-time of the built-in
switching transistor is so controlled that pin VOUT of
the S-8330 will assume an output voltage matched to
the voltage value which was preset via the electronic
volume. The circuit has its switching frequency fixed
and can be selected between two frequencies
available, namely, 50 kHz (S-8330B) or 180 kHz (S8330A). As a general rule, a frequency of 50 kHz is
chosen when the load current is lower than 1 mA, and
180 kHz is used when the said current is 1 mA or
higher.
The electronic volume can vary the output positive
voltage value in a narrow range around 12 V, with 8 bit
resolution. The products can be selected in steps of 2
V in a maximum output voltage range of 20 V to 30 V.
The electronic volume comprises an 8-bit shift
register and an 8-bit latch, as shown in Figure 2. It
Application to 15/-9 V power supply
Use of a transformer externally coupled to the S8330 makes it possible to obtain the positive and
negative voltages required for the SA and MLA
systems.
Figure 3 illustrates a 15/-9 V power supply circuit
designed to output positive and negative voltages in a
flyback configuration. The circuit basically functions in
this way: As the switching transistor built in the S-8330
(Msw) comes on, a current flows to the primary side of
the transformer (T1) and the energy is stored. When
the said transistor goes off, the secondary side and the
tertiary side of the transformer release energy
proportional to their winding ratio to the output, and
thus the positive or negative voltage is generated.
The output positive voltage value is stabilized by
connecting pin VOUT of the S-8330 to positive voltage
output +VOUT, while the output negative voltage value
is determined and stabilized by the winding ratio of the
secondary side and tertiary side of the transformer.
D
L
Murata/LQH4N/100uH
Matsushita/MA720
CONT
VOUT
S-8330B30
VIN
records the data of VDIN in the shift register at the
rising edge of VCLK, latches the data by briefly making
VSTRB High, and holds the data by making VSTRB
Low again. The electronic volume is reset when VCLK
= VSTRB = High, and is then set to the minimum
output voltage.
The power-off function is achieved making the
ON/OFF terminals of the S-8330 Low. This function
turns off the built-in switching transistor (Msw)
permanently and also switches off the internal circuit,
thereby cutting off the consumption current to almost
nil. Moreover, since the Nch transistor (Mdc) built in
the S-8330 comes on, the electric charge of the output
capacitor (CL) can be discharged to reduce the
positive voltage to 0 V as the external transistor (M1)
inserted between the diode (D) and the positive
voltage output is turned off. The sample circuit given
here uses a small-signal transistor (M2) and turns off
the external transistor (M1) in interlock with the Low
signal of the ON/OFF pin. The Zener diode (ZD) is
inserted there to clamp the voltage and avoid imposing
a voltage in excess of the withstand voltage between
the gate and source of the external transistor (M1).
M1
+VOUT
Positive Voltage
ZD
NEC/
02CZ15
VDIN
VCLK
2SJ356
8-Bit Shift
Register
VDIN
+
Cin
VCLK
-
Electronic
Volume
+
-
PW M
Control Circuit
RA:
500k Ω
MSW
Mdc
RB:
1M Ω
M2
VSTRB
ON/OFF
VIN
VREF
Setting
CR Oscillation Circuit
RESET
CL
- Nichicon/
F93/
100u
VSTRB
READ
HOLD
8-Bit
Latch
RESET
VSS
Figure 1. Basic Operation Circuit of the S-8330
Seiko Instruments Inc.
LCD
Panel
+
VOUT
Electronic
Volume
Figure 2. Electronic Volume Circuit
1
15/-9 V 8-Bit Programmable Power Supply Using the S-8330
When the numbers of turns of the secondary and
tertiary side windings of the transformer are n2 and n3,
respectively, the output negative voltage value -Vout is
determined by the following equation:
This means that output negative voltage value
-Vout and output positive voltage value +Vout maintain
a constant mutual interlock.
− V out = −
n3
× +V o u t
n2
VCONT =
Because the characteristics of the power supply
circuit being discussed here depend on the transformer
(T1), you will now look at the key points to be
considered in selecting the transformer. First, you
must decide the number of turns n1 on the primary
side of the transformer in such a manner that the
inductance value on the primary side will be covered in
a range from 22 to 100 µH, if the S-8330A is
employed, or from 47 to 220 µH, if the S-8330B is
used. In the example shown in Figure 3, the S-8330B
was used and the primary side inductance value was
set to some 100 µH.
When the built-in switching transistor comes on,
allowing a current to flow across the primary side of
the transformer, voltages V1 and V2, which are
expressed by Equations (2) and (3) below, will be
generated on the secondary and tertiary sides of the
transformer.
n2
× + V IN
n1
······(2)
V2 =
n3
× + V IN
n1
······(3)
Since these voltages V1 and V2 act on diodes D1
and D2, the product should be chosen with attention
paid to the reverse breakdown voltage of the diodes.
When the built-in switching transistor comes off,
allowing a current to flow across the secondary and
tertiary sides of the transformer, a voltage will be
generated on the primary side, and voltage VCONT,
which is expressed by Equation (4) below, will be
imposed on pin CONT of the S-8330.
Transformer
Sumida/CEE93
VOUT
VIN
S-8330B24
T1
ON/OFF
VCLK
Cin
VSTRB
Electronic
Volume
+
-
RESET
PW M
Control Circuit
CL1
+
Secondary
side
Nichicon/
20
-
F93/10u
CONT
Tertiary
side
12
VIN
CR Oscillation Circuit
Msw
D2
R1
100k
Set to 15 V
LCD
panel
Mdc
Primary
side 10
VREF
+VOUT
Positive voltage
D1
Matsushita/MA720
M1
VDIN
+
······(4)
You must then decide the ratio between the
number of turns n1 in the transformer primary side
winding and the number of turns n2 in the secondary
side winding in such a manner that the said voltage
VCONT will not exceed the 33-V withstand voltage of
pin CONT in the S-8330.
Here, care should be taken regarding possible
deviations in the winding ratio of the secondary and
tertiary sides of the transformer in particular, inasmuch
as it will sensibly be reflected in the deviations in the
output's negative voltage value.
Generally,
transformers have a gap in their cores. For this power
supply circuit, however, it is preferable to select a
transformer with as little gap as possible. This is
because a large gap in the core seems to accentuate
the adverse effects caused by ratio deviations resulting
from the way the windings are formed.
The circuit can be powered off when the ON/OFF
pin of the S-8330 are made Low. When the circuit is
powered off, the Nch transistor (Mdc) built in the S8330 comes on, prompting the positive voltage output
to fall to 0 V. The negative voltage output side can be
brought down to 0 V by adding an external circuit that
consists of small-signal transistors M1, M2, and M3,
and resistor R1.
This power supply circuit has limits in accuracy
because the associated transformer dictates the
accuracy of the ratio between the positive and
negative output voltages. Where a very high accuracy
is sought, the general practice is to create a negative
voltage by adding an inverting amplifier referenced to
0 V in between the positive and negative voltage
outputs of this circuit, or to add a 3-pin negativevoltage regulator to the negative voltage output. Other
approaches are also employed.
Figure 3 represents the settings aimed at attaining
high efficiency under the conditions of a 15/-9 V output
and 0.5 mA load.
The corresponding efficiency
characteristics are given in Figure 4.
······(1)
V1=
n1
× + Vout + VIN
n2
Matsushita/
MA720
VSS
M3
CL2
Nichicon/
F93/10u
+
-VOUT
Negative voltage
M2
Figure 3. 15/-9 V Power Supply Circuit
Seiko Instruments Inc.
2
15/-9 V 8-Bit Programmable Power Supply Using the S-8330
S-8330B
Transformer: Sumida CEE93 6375-002 (Non-gap) in use.
80%
70%
Efficiency
η (%)
60%
VIN=2.8V
VIN=3.8V
VIN=5V
50%
40%
0.01
0.1
1.
Output Current IOUT (mA)
10.
Figure 4
Seiko Instruments Inc.
3