PRODUCT DATASHEET
AAT2687
TM
SystemPower
PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Channel 1 Regulator
Output Capacitor Selection
Applications Information
Output 1 is a high voltage DC/DC step-down converter
providing an output voltage from 1.5V to 85% of the
minimum input voltage (85% · VIN). The integrated high-
side n-channel MOSFET device provides up to 4.5A out-
put current. Input voltage range is 6.0V to 24.0V. The
step-down converter utilizes constant frequency (PWM-
mode) voltage mode control to achieve high operating
efficiency while maintaining extremely low output noise
across the operating range. High 490kHz (nominal)
switching frequency allows small external filtering com-
ponents; achieving minimum cost and solution size.
External compensation allows the designer to optimize
the transient response while achieving stability across
the operating range.
Three 22μF ceramic output capacitors are required to
filter the inductor current ripple and supply the load
transient current for IOUT = 4.5A. The 1206 package with
10V minimum voltage rating is recommended for the
output capacitors to maintain a minimum capacitance
drop with DC bias.
Channel 1 Output Inductor Selection
The step-down converter utilizes constant frequency
(PWM-mode) voltage mode control. A 4.7ꢀH inductor
value is selected to maintain the desired output current
ripple and minimize the converter’s response time to
load transients. The peak switch current should not
exceed the inductor saturation current, the MOSFET or
the external Schottky rectifier peak current ratings.
Output 2 is a low voltage, low dropout (LDO) linear
regulator providing 1.8V with up to 600mA output cur-
rent. The input voltage range is 2.7V to 5.5V. The LDO
provides very low noise output which can be derived
directly from the Output 1 channel.
Channel 1 Rectifier Selection
When the high-side switch is on, the input voltage will be
applied to the cathode of the Shottky diode. The rectifi-
er's rated reverse breakdown voltage must be chosen at
least equal to the maximum input voltage of the step-
down regulator.
Channel 1 Output Voltage and Current
Output 1 is set using an external resistor divider as
shown in Table 1. Minimum output voltage is 1.5V and
maximum output voltage is 5.5V. Typical maximum duty
cycle is 85%.
When the high-side switch is off, the current will flow
from the power ground to the output through the
Schottky diode and the inductor. The power dissipation
of the Schottky diode during the time-off can be deter-
mined by the following equation:
R4 = 1.96kΩ
VOUT (V)
R3 (kΩ)
1.5
1.8
1.85
2.0
2.5
3.0
3.3
5.0
2.94
3.92
4.02
4.53
6.19
7.87
8.87
14.3
VOUT
PD = IOUT · VD · 1 -
VIN
Where VD is the voltage drop across the Schottky diode.
Channel 1 Input Capacitor Selection
Table 1: Feedback Resistor Values.
For low cost applications, a 220μF/25V electrolytic
capacitor is selected to control the voltage overshoot
across the high side MOSFET. A small ceramic capacitor
with voltage rating at least 1.05 times greater than the
maximum input voltage is connected as close as possible
to the input pin (Pin 14) for high frequency decoupling.
Alternatively, the feedback resistor may be calculated
using the following equation:
(VOUT - 0.6) · R4
R3 =
0.6
R3 is rounded to the nearest 1% resistor value.
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2687.2008.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
