AAT1415 Datasheet, PDF (26/32 Pages) Advanced Analog Technology, Inc. – FIVE-CHANNEL DC-DC CONVERTER WITH A 2.5V LDO

Advanced Analog Technology, Inc.
May 2008
AAT1415/AAT1415A
DESIGN PROCEDURE
Programming the Output Voltage
The output voltage for each channel is programmed
using a resistor divider from the output connected to
the feedback pins. When setting the output voltage,
connect a resistive voltage divider from the channel
output to the corresponding IN_ input and then to GND.
Choose the lower-side (IN_-to-GND) resistor, then
calculate the upper-side (output-to-IN_) resistor as
follows:
R_{UPPER _}
= R_{LOWER _}


V_{OUT _}
V_{IN_}

− 1
(For Boost / buck),
R_{UPPER _}
=
R_{LOWER _}


− V_{OUT _}
V_REF


(For Inverting),
Where V_{IN_} is the feedback regulation voltage, 1.250V
(AAT1415/AAT1415A, V_IN3 = 0.8V ), and typical values
for R_{LOWER _} are in the range of 10 kΩ to 100 kΩ .
Inductor Selection
The inductor is typically selected to operate with
continuous conduction mode (CCM) for best efficiency
in boost or buck converter and discontinuous
conduction mode (DCM) for better response ability in
boost or inverting controller (Table 1 and Table 2). The
recommended inductance value range is between
2.2µH and 4.7µH for boost. The recommended
inductance value range is between 6.8µH and 22µH for
buck. The recommended inductance value range is
between 3.3µH and 6.8µH for Inverting. With the
chosen inductance value, the peak current for the
inductor in steady state operation can be calculated
(Table 3). It also needs to be taken into account that
load transients and error conditions may cause higher
inductor currents. This also needs to be taken into
account when selecting an appropriate inductor.
Table 1. Response Ability for Various Topologies
Topology
Response Ability
Boost or
V_IN
Inverting
L
Buck
V_IN − V_OUT
L
V_IN : Input Voltage, V_OUT : Output Voltage,
L : Inductance, Response Ability Unit: ^mA
µs
Figure 9a. Feedback Network (For Boost/Buck)
Table 2. DCM/CCM Critical Inductance Values
Topology
D
DCM/CCM
Boost
1− ^VIN
V_OUT
R_LOAD⋅ D ⋅ (1− D)²
2 ⋅ f_SW
Buck
V_OUT
V_IN
(1− D) ⋅ R_LOAD
2 ⋅ f_SW
Inverting
V_OUT
V_OUT + V_IN
R_LOAD⋅ (1− D)²
2 ⋅ f_SW
Figure 9b. Feedback Network (For Inverting)
V_IN : Input Voltage, V_OUT : Output Voltage,
R_LOAD : Loading, f_SW : Switch Frequency,
Inductance Unit: Henry
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Version 3.00
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