PRODUCT DATASHEET
AAT2688
TM
SystemPower
4.5A PMIC Solution for 12V Adapter Systems with 2-Output 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 0.8V to 5.5V. The inte-
grated high-side N-channel MOSFET device provides up
to 4.5A output current. Input voltage range is 6.0V to
24.0V. The step-down converter utilizes constant fre-
quency (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 components, achieving minimum cost
and solution size. External compensation and an option-
al feed forward capacitor 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 4.7ꢁH 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 or the MOSFETs.
Output 2 is a low voltage low dropout (LDO) linear regu-
lator providing 1.8V with up to 600mA output current.
The input voltage range is 2.7V to 5.5V. The LDO pro-
vides very low noise output which can be derived direct-
ly from Output 1.
Channel 1 MOSFET Selection
The step-down (buck) converter utilizes synchronous
rectification (Q1) for constant frequency (PWM mode)
voltage mode control. The synchronous rectifier is select-
ed based on the desired RDS(ON) value and QG (total gate
charge), these two critical parameters are weighed
against each other. To get a low RDS(ON) value, the MOSFET
must be of a very large size and a larger MOSFET will
have a large QG. Conversely to get a low QG, the MOSFET
must be small and thus have a large RDS(ON) value. In
addition to the trade off between RDS(ON) and QG, the
maximum voltage rating for the external synchronous
MOSFET must exceed the maximum application input
voltage value (VDS[MAX] > VIN[MAX]).
Output Voltage—Channel 11
The output voltage is set using an external resistor
divider as shown in Table 1. Minimum output voltage is
0.8V and maximum output voltage is 5.5V. Typical max-
imum duty cycle is 85%. Example: with R4 = 1.96Kꢀ,
(VOUT - 0.585) · R4
R3 =
0.585
1
VOUT (V)
R3 (kΩ)
The QG affects the turn-on/turn-off time of the synchro-
nous MOSFET, the longer the turn-on/turn-off time the
more likely the step-down converter will have “shoot
through” current issues. “Shoot through” current occurs
when the AAT2688 internal top-side MOSFET and the
external synchronous MOSFET are conducting current at
the same time. This will result in a low impedance path
to ground from the input voltage through the two
MOSFETs, and the current may exceed the maximum
current rating of the AAT2688 and external synchronous
MOSFET. Exceeding the maximum current ratings will
lead to the destructive derating of the AAT2688 and
external synchronous MOSFET.
0.8
1.0
1.2
1.5
1.8
2.0
2.5
3.0
3.3
5.0
0.715
1.37
2.05
3.09
4.02
4.75
6.49
8.06
9.09
14.7
Table 1: External Resistor Values (Standard 1%
Resistors are Substituted for Calculated Values).
1. The R3 and R4 feedback resistors are separate from the compensation network. When changing either R3 and/or R4, the compensation network will have to be altered. Contact
the Applications Engineering department for compensation network recommendations for specific output voltages.
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2688.2008.06.1.0
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