FAIRCHILD AN-8023

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AN-8023
Negative Voltage Management Using a
FAN8303 Buck Regulator
Abstract
FAN8303 is a 2A, 370kHz monolithic integrated buck
regulator with internal power MOSFETs. It is simple to use
and needs minimal external components. This application
note describes how to generate negative voltage using
FAN8303. It introduces application examples and discusses
optimized designs for a buck-boost circuit.
Introduction
Buck regulators are widely used for higher voltage to lower
voltage DC conversion. Likewise, FAN8303 was originally
designed for application needing regulated DC voltage, such
as set-top box microcontrollers and efficient pre-regulators
Figure 1 shows a practical application of an LCD panel; it
needs negative voltage for contrast control. In this block
diagram, a charge pump is usually adopted due to the simple
design and low cost. However, it has an amount of power
dissipation and poor output voltage regulation relative to
input voltage variation. FAN8303 with negative output
would be a solution to overcome these problems.
12V
2.5V, 3.3V
LDO
DC/DC (buck)
SoC
2.5V
DDR2
EEPROM
Row Drivers
-5V , 15V
Charge pump
AV Board
for linear regulators in PC monitor and TV applications. In
some cases, a non-synchronous buck regulator also can be
utilized for buck-boost circuit to generate negative voltage
with respect to ground. These applications include audio
amplifier, timing control circuit for LCD panel, and so on.
TFT LCD Panel
DC/DC (boost)
Column Drivers
16V
P-gamma IC
Figure 1. Example of Timing Control Block
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
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AN-8023
APPLICATION NOTE
Principle of Operation
switch Q1 (Figure 2) is turned on, VL is same as VIN, so IL
ramps up with VIN/L. During the Q1 off-time, VL has reverse
polarity to maintain continuous inductor current with -VOUT.
Therefore, it can generate negative output voltage.
To understand buck-boost topology, buck topolgy is briefly
compared below. When the MOSFET switch (Q1 in Figure 3)
is turned on, the voltage across inductor (VL) is VIN -VOUT.
During Q1 off-time, VL is equal to -VOUT in buck topology. So
the inductor current (IL) ramps up with (VIN-VOUT)/L and
ramps down with VOUT/L slope. Thus, the energy can be
transferred to the load with positive output voltage.
Meanwhile, in buck-boost topology, the inductor and
freewheeling diode switch positions. When the MOSFET
Vo
D1
Q1
Buck-boost circuit with buck regulators require several
design considerations. Table 1 summarizes the design
parameter comparison between buck and buck-boost circuit.
PWM
VIN
CIN
LOUT
COUT
LOUT
Q1
Vo
PWM
Load
VIN
D1
CIN
COUT
Load
Q1 ON
Q1 OFF
Q1 ON
Q1 OFF
I Q1
I Q1
t
t
I D1
I D1
IL
IL
∆I L
∆I L
VL
VL
VIN
VIN − VOU T
VOUT
−VOUT
−VOUT
VOUT
Figure 2. Buck-Boost Topology
Table 1.
Figure 3. Buck Topology
Buck and Buck-Boost Design Parameters
Topology
IL (Average)
Maximum VSW
Duty Cycles
Buck-Boost
IOUT
1− D
VIN + VOUT
VOUT
VIN + VOUT
Buck
I OUT
VIN
VOUT
VIN
limited to the maximum switch node voltage of buck
regulator. Since buck-boost is very noisy on input and
output compared to buck circuit, it requires good-quality
MLCC as input and output filters.
First of all, inductor current is limited by (1–D); so attention
is needed to see that the maximum output current of buck
regulator is be always lower than the maximum current in
buck-boost circuit. Second, the switch node is a sum of input
voltage and output voltage in buck-boost. It also needs to be
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
www.fairchildsemi.com
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AN-8023
APPLICATION NOTE
Design Considerations
Inductor Selection
Input Capacitor
When choosing inductor, the main concerns are inductance
value, RMS current rating, and DCR. Inductance value is
usually adopted higher than the minimum inductance to
operate Continuous Current Mode (CCM). RMS current
should be higher than the inductor current to prevent inductor
saturation without core loss. A low-DCR inductor is usually
adopted when a power system needs high efficiency.
The input capacitor should handle the maximum input RMS
current, so use the equations below for calculation. Good
estimation is given by 10µF or 22µF per amp with MLCC.
Maximum RMS input current:
IRMS _ MAX = IOUTMAX ×
VIN × D
fSW × ΔIL
CMIN = (IRMS × D) / (fSW × ΔVIN )
Freewheeling Diode
VOUT
VOUT + VIN
The freewheeling diode acts as a inductor current path when
the switch is turned off. Breakdown voltage, lower forward
drop voltage, and the maximum current rating are
considered for low power dissipation. A Schottky diode is
preferred, which has low forward voltage drop.
= Duty cycle;
fSW
= Switching frequency; and
ΔIL
= Ripple current to maintain continuous
current mode (typically 20%~30% of IL).
Required diode current rating:
> ILMAX
Output Capacitor
IOUTMAX × DMAX
fSW × ΔVOUT
(6)
where ILMAX is maximum inductor current.
An output capacitor is needed to satisfy the output voltage
ripple requirement and to maintain constant output voltage
during dynamic load condition. Ripple voltage depends on
ESR, output capacitance, and ESL. To obtain the desired
output ripple, the below equation for required minimum
capacitance is useful:
CMIN =
(5)
where ΔVIN is desired input voltage ripple.
(1)
where:
D=
(4)
Required minimum capacitance:
To operate in continuous current mode, critical minimum
inductance is calculated by:
L=
(D × (1 − D))
Required breakdown voltage:
> VIN + VOUT
(7)
(2)
where:
DMAX
=
Maximum Duty Cycle;
IOUTMAX =
Maximum Output Current; and
ΔVOUT
Desired Output Voltage Ripple.
=
The equation for required ESR is:
ESR =
ΔVOUT
ILMAX
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
(3)
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AN-8023
APPLICATION NOTE
Design Example
A design example with test conditions VIN =12V, VOUT =
-5V, IOUT = 1A, and fSW =370 kHz (fixed) is shown below.
The first step is to set the critical design parameters, such as
inductor ripple current (∆IL) and desired output ripple
voltage (∆VOUT). The second step is calculation of duty
cycle. To achieve accurate value, consider the forward
voltage drop of diode and MOSFET switch on drop voltage.
Table 2.
Fairchild FAN8303, non-synchronous buck regulator has
integrated 0.22Ω N-channel MOSFET, so on drop voltage is
about 0.4V. Forward voltage of the Schottky diode
(40VRRM/ 2A IOUT) is 0.45V. When it comes to the inductor,
a higher value than calculated is recommended and a low
DCR inductor is preferred:
Design Example Calculations
Duty Cycle:
= (|VOUT|+ VF) / (VIN +|VOUT|+VF-VQ1)
0.33
Inductance:
= (VIN × D)/ (fSW × ∆IL)
35.6µH (desired ∆IL = 20%)
Output Capacitance:
= (IOUT × D) / (fSW × ∆VOUT)
86.8µF (desired ∆VOUT = 10mV)
Input Capacitance:
IRMS = IOUT × D × (1 − D)
0.47A
CIN = IRMS × D/ (∆VIN ×fSW )
4.05µF
Diode Current Rating:
IDIODE_MAX = IAVG + ∆IL/2
1.77A
where IAVG = Average Inductor Current
CBS
10nF
INPUT
12V
BOOT
Q1
VIN
L1
39µH
VSW
R2
FB
PWM
GND
CIN
10µF
CIN2
10µF
GND
2.45k
COMP
SS
D1
CC
1nF
CSS
10nF
R3
COUT
22µF x 4EA
Load
18k
RC
40k
OUTPUT
-5V / 1A
Figure 4. Buck-Boost Schematic Using FAN8303
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
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AN-8023
APPLICATION NOTE
Typical Waveforms & Graphs
Figure 7 shows FAN8303 efficiency and power-loss graph.
It indicates a maximum of 87% efficiency with 0.31W at
400mA load condition.
Figure 5 and Figure 6 show the typical waveforms of the
FAN8303 output ripple voltage. To achieve low ripple
voltage, lower than 10mΩ MLCC is used.
VOUT, 50mV/div
VOUT, 50mV/div
IOUT, 500mA/div
IOUT, 500mA/div
VSW, 5V/div
VSW, 5V/div
Figure 5. VOUT Ripple (1µs/div), 33mV at 100mA
Figure 6. VOUT Ripple (1µs/div), 89mV at 1A
Note:
1.
Test conditions: VIN =12V, VOUT = -5V, fSW = fixed 370 kHz, and IOUT = 0~1A.
Efficiency and Power Loss
88
1.0
86
0.9
0.8
0.7
82
0.6
80
0.5
78
0.4
76
0.3
Efficiency
74
Power loss
72
Power Loss (W)
Efficiency (%)
84
0.2
0.1
70
0.0
0
200
400
600
800
1000
Load (mA)
Figure 7. Efficiency and Power Loss
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
www.fairchildsemi.com
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AN-8023
APPLICATION NOTE
Conclusion
FAN8303 also can be utilized for buck-boost circuit to
generate negative output voltage with simple changes of
passive element.
Fairchild 2A monolithic and non-synchronous buck
regulator, FAN8303, has wide input range (~23V) with
excellent load and line regulation. In spite of buck regulator,
Author
DSEOM Application Engineer, SGYOON Application Engineer
Related Datasheets
FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator
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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.
As used herein:
1.
Life support devices or systems are devices or systems which,
(a) are intended for surgical implant into the body, or (b) support
or sustain life, or (c) whose failure to perform when properly
used in accordance with instructions for use provided in the
labeling, can be reasonably expected to result in significant
injury to the user.
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/30/09
2.
A critical component is any component of a life support device
or system whose failure to perform can be reasonably expected
to cause the failure of the life support device or system, or to
affect its safety or effectiveness.
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