AAT1160 Datasheet, PDF (10/14 Pages) Advanced Analog Technology, Inc. – Step Up Current Mode PWM Converter with 4+1 Operational Amplifiers

Advanced Analog Technology, Inc.
Operation Information:
AAT1160
Setting the Output Voltage:
Output voltage is set by using a feedback pin and a
resistor divider; equation is as follows:
V_P1 =1.265 (1+R1/R2)
Soft-Start:
The soft-start function is embedded with a typical
setting of 14ms
Operation Frequency Setting:
Pin4 (FREQ) can set PWM operation to low with
f_OSC =600kHz, or set PWM operation to high with
f _OSC =1.2 MHz.
Diode Selection:
Schottky diode for the boost regulator must be
selected correctly depending on the output voltage
and the output current. The diode must have a
reverse voltage equal to or greater than the output
voltage. In addition, the diode current must
exceed the switch current limit as a lower forward
voltage will increase efficiency.
Inductor Selection:
I _L(peak) = I _IN + V_IN *D/(2*L* f_S )
D: PWM duty
V_O / V_IN =1/ (1-D)
Maximum current of the inductor must be greater
than I _L(peak)
The Compensation:
Pin17 (EO) series with R _C & C_C for
compensation, usually range as 1k ≤ R _C ≤ 200k
, 100pF ≤ C_C ≤ 6,800pF. Standard formulas are
only for reference purpose, as PCB has a lot of
noise or parasitic capacitance and resistance, it
often requires on board adjustment.
The key steps for step-up compensation are as
follows:
Transconductance (from FB to CC), g_m (105µS)
Current-sense amplifier transresistance, R _CS
(0.275V/A)
For continuous conduction, the right-half-plane
zero frequency ( f _RHPZ ) is given by the following:
f _RHPZ = V_O (1-D)²/(2 *L* I _LOAD ) where D=the
duty cycle=1-( V_IN / V_O ), L is the inductance value,
and I _LOAD is the maximum output current. Typical
target, crossover ( f _C ), is 1/6 of the RHPZ.
For example, if oscillation frequency is assumed to
be f _OSC =500kHz, V_IN =2.5V, V_O =5V, and
I_OUT =0.5A, then R _LOAD =10 . If we select
L=4.7µH, then:
f _RHPZ =5(2.5/5)²/(2 *4.7*10^-6*0.5)=84.65kHz
If f _C =14kHz, C_C would be calculated as:
C_C =( V_FB / V_O )( R _LOAD / R _CS )( g _m /2π* f _C )(1-D)/2=(
1.25/5)(10/0.275)*[105µS/(6.28*14kHz)](1/4)=2.7
nF
Now selected R _C has fulfilled transient-droop
requirements. For example, if 4% of transient
droop is allowed, input to error amplifier moves
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– Advanced Analog Technology, Inc. –
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