AAT2689 Datasheet, PDF (11/20 Pages) Advanced Analogic Technologies – PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters

PRODUCT DATASHEET
AAT2689
SystemPower^TM PMIC Solution for 12V Adapter Systems with 2-Output High Performance Step-Down Converters
Applications Information
Output 1 is a high voltage DC/DC Buck (step-down) con-
verter providing an output voltage from 1.5V to 5.5V.
The integrated high-side N-channel MOSFET device pro-
vides up to 2.5A output 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 components, achieving minimum cost
and solution size. External compensation allows the
designer to optimize the transient response while achiev-
ing stability across the operating range.
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.
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%.
V_OUT(V)
1.5
1.8
1.85
2.0
2.5
3.0
3.3
5.0
R₄ = 6.04kΩ
R₃(kΩ)
9.09
12.1
12.4
14.0
19.1
24.3
27.4
44.2
Table 1: Feedback Resistor Values
Alternately, the feedback resistor may be calculated
using the following equation:
R₃
=
(V_OUT
- 0.6)
0.6
·
R₄
R3 is rounded to the nearest 1% resistor value.
Channel 1 Regulator
Output Capacitor Selection
Two 22μF ceramic output capacitors are required to filter
the inductor current ripple and supply the load transient
current for I_OUT = 2.5A. 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 4.7μH 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.
Channel 1 Rectifier Selection
When the high-side switch is on, the input voltage will be
applied to the cathode of the Schottky diode. The recti-
fier's rated reverse breakdown voltage must be chosen
at least equal to the maximum input voltage of the step-
down regulator.
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:
P_D = I_OUT · V_D ·
1 - ^VOUT
V_IN
Where V_D is the voltage drop across the Schottky diode.
Channel 1 Input Capacitor Selection
For low cost applications, a 100μF/25V electrolytic capac-
itor 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 max-
imum input voltage is connected as close as possible to
the input pin (Pin 14) for high frequency decoupling.
2689.2008.06.1.0
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