Advanced Analog Technology, Inc
.
May 2008
AAT1168/1168A/1168B
voltage and input voltage of 5V. Choose a κ of 0.465
Input Capacitor Selection
and efficiency of 90%.
The input capacitors have two important functions in
PWM controller. First, an input capacitor provides the
power for soft start procedure and supply the current
0.9 *13.3
2
6.8µH
L ≥
0.624(0.376) ≈
6
0.465 * 0.3 *1.1
I
O
for the gate-driving circuit. A 10µF ceramic capacitor
I
=
= 0.89A
IN
η(1− D)
is used in typical circuit. Second, an input bypass
capacitor reduces the current peaks, the input voltage
drop, and noise injection into the IC. A low ESR
V D
IN
I
= I
+
= 1.095A
L
IN
(peak)
2Lf
s
= 0.043W or 1% power loss
PDCR
ceramics capacitor 0.1
µ
F
is used in typical circuit. To
VDD is decoupled
ensure the low noise supply at VDD
,
Schottky Diode Selection
from input capacitor using an RC low pass filter.
Schottky has to be able to dissipate power. The
dissipated power is the forward voltage and input DC
current. To achieve the best efficiency, choose a
Schottky diode with less recovery capacitor (CT) for
fast recovery time and low forward voltage (VF).
For boost converter, the reverse voltage rating (VR)
should be higher than the maximum output voltage,
and current rating should exceed the input DC current.
=
PDIODE
PDSW + PDCOM
(1−D)VFQRfs
P
=
DSW
Figure 3. Input Bypass Capacitor Affects the
Drop
V
DD
QR
= VRCT
PDCOM = VFIO /(1− D)
Output Capacitor
The output capacitor maintains the DC output voltage.
A Low ESR ( C ) ceramic capacitor can reduce the
: Total power loss of diode for boost converter
PDIODE
: Switching loss of diode for boost converter
PDSW
r
: Conduction loss of diode for boost converter
PDCOM
output ripple and power loss. There are two
parameters which can affect the output voltage ripple:
1. the voltage drops when the inductor current flows
through the ESR of output capacitor; 2. charging and
discharging of the output capacitor also affect the
output voltage ripple.
Table 2. Schottky ata ist
V
V
C
T
SMA
F
R
B220A
B240A
0.24V
0.24V
14V
28V
150pF
150pF
VRIPPLE
=
VRIPPLE(COUT
IOD
fSCOUT
) + VRIPPLE(ESR)
DIODES Product, Max-Height: 2.3mm
VRIPPLE(COUT ) ≈
For example,
PDIODE = PDSW + PDCOM
V
(ESR) ≈ I
r
L(peak) C
RIPPLE
= 0.203W or 5.1% power loss.
–
–
–
Advanced Analog Technology, Inc. –
Version 1.00
Page 15 of 22
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
