PRODUCT DATASHEET
AAT1189
TM
SwitchReg
High Voltage Step-Down Regulator
capacitors to maintain a minimum capacitance drop with
DC bias.
Applications Information
The high voltage DC/DC step-down converter provides
an output voltage from 1.5V to 5.5V. The integrated
high-side n-channel MOSFET device provides up to 2.5A
output current1. Input voltage range is 6.0V to 24.0V.
The step-down converter utilizes constant frequency
(PWM-mode) voltage mode control to achieve high oper-
ating 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.
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.
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.
Output Voltage and Current
The output voltage 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 max-
imum 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:
R5 = 6.04kΩ
VOUT(V)
R4(kΩ)
1.5
1.8
1.85
2.0
2.5
3.0
3.3
5.0
9.09
12.1
12.4
14.0
19.1
24.3
27.4
44.2
VOUT
PD = IOUT · VD · 1 -
VIN
Where VD is the voltage drop across the Schottky diode.
Input Capacitor Selection
Table 1: Feedback Resistor Values.
For low cost applications, a 100μF/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) · R5
R4 =
0.6
Feedback and Compensation Networks
R4 is rounded to the nearest 1% resistor value.
The transfer function of the Error Amplifier is dominated
by the DC Gain and the L COUT output filter of the regula-
tor. This output filter and its equivalent series resistor
(ESR) create a double pole at FLC and a zero at FESR in the
following equations:
Buck Regulator Output
Capacitor Selection
Two 22μF ceramic output capacitors are required to filter
the inductor current ripple and supply the load transient
current for IOUT = 2.5A. The 1206 package with 10V
minimum voltage rating is recommended for the output
1
F
=
Eq. 1:
LC
2 · π · L · COUT
1. Output current capability may vary and is dependent on package selection, maximum ambient temperature, airflow and PCB heatsinking.
w w w . a n a l o g i c t e c h . c o m
1189.2008.06.1.0
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