AAT1168 Datasheet, PDF (15/22 Pages) Advanced Analog Technology, Inc. – TRIPLE-CHANNEL TFT LCD POWER SOLUTION WITH OPERATIONAL AMPLIFIERS

Advanced Analog Technology, Inc.
May 2008
AAT1168/1168A/1168B
voltage and input voltage of 5V. Choose a κ of 0.465
and efficiency of 90%.
L ≥ ^{0.9 * 13.3} 0.624(0.376)² ≈ 6.8 µH
0.465 * 0.3 * 1.1⁶
I_IN
=
I_O
η(1− D)
=
0.89A
I_{L(peak )}
= I_IN
+
V_IND
2Lf_s
=
1.095A
P_DCR = 0.043W or 1% power loss
Schottky Diode Selection
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 (V_R)
should be higher than the maximum output voltage,
and current rating should exceed the input DC current.
Input Capacitor Selection
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
for the gate-driving circuit. A 10 µF ceramic capacitor
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
ceramics capacitor 0.1 µF is used in typical circuit. To
ensure the low noise supply at V_DD , V_DD is decoupled
from input capacitor using an RC low pass filter.
P_DIODE = P_DSW + P_DCOM
P_DSW = (1− D)V_FQ_Rf_s
Q_R = V_RC_T
P_DCOM = V_FI_O /(1− D)
Figure 3. Input Bypass Capacitor Affects the V_DD
Drop
P_DIODE : Total power loss of diode for boost converter
P_DSW : Switching loss of diode for boost converter
P_DCOM : Conduction loss of diode for boost converter
Output Capacitor
The output capacitor maintains the DC output voltage.
A Low ESR ( r_C ) ceramic capacitor can reduce the
output ripple and power loss. There are two
Table 2. Schottky Data List
SMA
V_F
V_R
C_T
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
B220A
B240A
0.24V 14V 150pF
0.24V 28V 150pF
DIODES Product, Max-Height: 2.3mm
For example,
P_DIODE = P_DSW + P_DCOM = 0.203W or 5.1% power loss.
discharging of the output capacitor also affect the
output voltage ripple.
V_RIPPLE = V_RIPPLE (C_OUT ) + V_RIPPLE (ESR)
V_RIPPLE (C_OUT )
≈
I_OD
f_SC_OUT
V_RIPPLE(ESR) ≈ I_L(peak)r_C
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