AAT1415 Datasheet, PDF (30/32 Pages) Advanced Analog Technology, Inc. – FIVE-CHANNEL DC-DC CONVERTER WITH A 2.5V LDO

Advanced Analog Technology, Inc.
May 2008
AAT1415/AAT1415A
C_C
=
1.25V
250mA × 0.5V
/
A
×
70µ A / V
2π ⋅ 70kHz
≈
2.2nF ,
R_C
=
V_OUT ⋅ R_LOAD ⋅ C_OUT
(2 ⋅ V_OUT − V_I ) ⋅ C_C
R_C
= 0.325A ×
0.5V / A
0.03 ×1.25 × 70µ A
/
V
≈ 68kΩ ,
C_OUT
=
250mA
1.5V
× 68kΩ × 2.2nF
≈ 22µF ,
C_U
=
1
2π × 20kΩ × 70kHz × (1.25V1.5V)
≈
220pF .
Boost Controller Compensation
The boost controller employs voltage-mode control to
regulate their output voltage. A benefit of discontinuous
conduction mode (DCM) is more flexible loop
compensation, better response ability and no maximum
duty-cycle restriction on boost ratio. When the boost
converter operates with discontinuous conduction
mode (typically the CCD VH case), the boost controller
has a single pole at the following:
f_P
=
2 × V_OUT − V_I
2π × R_LOAD × C_OUT × V_OUT
Set the loop cross over frequency ( f_C ) below the lower
of 1/10 the switching frequency ( f_SW ). Choose the
compensation capacitor C_C to set the desired
crossover frequency f_C . Determine the value by the
following equation:
C_C
=
V_OUT ⋅ V_{I ×}
(2 ⋅ V_OUT − V_I )
G_m
2π ⋅ f_C
×
V_OUT
M ⋅ (V_OUT − V_I )
Where:
M = ^{2 ⋅ L ⋅ fSW}
R_LOAD
The R_CC_C zero is then used to cancel the f_P pole,
so:
The typical R_C is under 500 kΩ .
If the system wants better transient response, it can
parallel a capacitor C_U with R_{UPPER _} from IN_ to
V_{OUT _} :
C_U
=
2π × R_{UPPER _}
1
× f_C
×


V_IN
_

V_{OUT _}


If C_U is calculated to be less than 10pF, it can be
omitted. Additionally, C_U can suppress the inrush
current
So, for a 13V/30mA output with V_I = 3.0V, L = 3.5 µH ,
R_UPPER = 100 kΩ , f_SW = 500kHz:
C_C
=
13V ⋅ 3V
(2 ⋅13V − 3V)
×
70µ A / V
2π ⋅ 50kHz
×
13V
8m ⋅ (13V − 2V)
≈ 4.7nF
R_C
=
13V ⋅ 433Ω ⋅10µF
(2 ⋅13V − 3V) ⋅ 4.7nF
≈
500kΩ
,
( ) ^CU
=
1
2π ×100k × 50kHz ×
1.25V
13V
≈ 330pF .
Inverting Controller Compensation
The inverting controller also employs voltage-mode
control to regulate their output voltage. To operate in
discontinuous conduction mode (DCM) is preferred for
simple loop compensation and freedom from duty-cycle
restrictions on the inverter input-output ratio. When the
Inverting converter operates with discontinuous
conduction mode (typically the CCD VL case), the
inverting controller has a single pole at the following:
f_P
=
1
π ⋅ R_LOAD
⋅ C_OUT
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