AAT1415 Datasheet, PDF (31/32 Pages) Advanced Analog Technology, Inc. – FIVE-CHANNEL DC-DC CONVERTER WITH A 2.5V LDO

Advanced Analog Technology, Inc.
May 2008
AAT1415/AAT1415A
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
+ V_REF ) ⋅
× ^Gm
M ^{2π ⋅ f}_C
Where:
M = ^{2 ⋅ L ⋅ fSW}
R_LOAD
The R_CC_C zero is then used to cancel the f_P pole,
so:
R_C
=
R_LOAD ⋅ C_OUT
2 ⋅ C_C
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
=
1

2π
×
56kΩ
×
20kHz
×



V_{IN _}
V_{OUT _} + V_{IN _}




≈ 1nF
LAYOUT CONSIDERATIONS
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. The compensation network should be very
close to the EO_ pin and avoid through VIA. The IC
must be bypassed with ceramic capacitors placed
close to the VDD and VREF. A separate low-noise
ground plane containing the reference and signal
grounds should connect to the power-ground plane at
only one point to minimize the effects of power-ground
currents. Typically, the ground planes are best joined
right at the IC. Tie the feedback resistor divider to be
very close to output capacitor and far away from the
inductor or Schottky diode. Keep the feedback network
(IN_) close to the IC. Switching nodes (SW_) should be
kept as small as possible and should be routed away
from high-impedance nodes such as IN.
C_U
=
2π × R_{UPPER _}
1

×
f_C
×



V_{IN _}
V_{OUT _} + V_{IN _}




If C_U is calculated to be less than 10pF, it can be
omitted. Additional C_U can suppress the inrush current.
So, for a –7V/50mA output with V_I = 3.0V, L = 4.7µH,
R_UPPER = 56 kΩ , f_SW = 500kHz:
C_C
=
( −7
3V
+ 1.25V) ⋅
× ^{70µ A / V} ≈ 2.2nF
33.6m ^{2π ⋅ 20kHz}
R_C
=
140Ω ⋅10µF
2 ⋅ 2.2nF
≈
300kΩ
,
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– Advanced Analog Technology, Inc. –
Version 3.00
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