AAT1187A Advanced Analog Technology, Inc., AAT1187A Datasheet - Page 17

• Manufacturer Part Number: AAT1187A
• Description: Boost Converter For Wled Driver
• Manufacturer: Advanced Analog Technology, Inc.

DETAILED DESCRIPTION
The AAT1187A is a compact power solution designed
for TFT LCD panels. It consists of a current mode
boost pulse-width modulation, positive charge pump
regulator, rail-to-rail buffer, low voltage high driving
LDO, GPM function, and reset function.
Boost DC-DC Converter
The inductor undergoes current rise during on time.
When the inductor current reaches a current tri-point
predetermined by internal error amplifier, power MOS
turns off, and energy stored in inductor is released to
boost regulated output voltage. The internal error
amplifier output (EO) determines the necessary current
amount to support the load.
Frequency Selection
The AAT1187A has a frequency selection pin, which
allows users to set operational frequency to 640kHz or
1.2MHz. Connecting FREQ pin to VDD, users may
operate the device at 1.2MHz switching frequency. In
addition, the boost regulated output voltage also has a
lower output ripple voltage. When working with a
640kHz switching frequency, simply connect FREQ pin
to ground or leave it floating as FREQ pin is equipped
with internal 5µA pull down current.
Under Voltage Lockout
To avoid abnormal operation at lower VDD voltage, the
under voltage lockout function (UVLO voltage 2.1V)
monitors the controller. When VDD falls bellow UVLO
voltage, AAT1187A disables functions including boost
pulse-width modulation function, positive charge pump
regulator, GPM, and buffer function. LDO, bandgap
reference, and reset function remain enabled.
Advanced Analog Technology, Inc.
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Advanced Analog Technology, Inc . –
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Page 17 of 28
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Fault Protection
Fault protection in AAT1187A includes the following:
OVC :
OVC is a cycle-by-cycle current limit in boost controller,
which limits peak inductor current UVP. When boost
feedback voltage and positive charge pump feedback
voltage falls under 85% of the default setting,
AAT1187A activates timer. If UV condition remains at
160ms, AAT1187A launches fault latch.
OTP (Over Temperature Detection):
When
AAT1187A activates timer. If OTP condition remains
for 160ms, AAT1187A launches fault latch.
Setting the Boost Soft-Start
The AAT1187A has an adjustable soft-start function to
prevent high inrush current during start up. The
soft-start function can be implemented with an external
capacitor with a 4µA constant current.
When V
constant current charges an external capacitor C
When SS voltage is higher than the V
boost controller starts and rises to V
Soft-start function would be completed when SS
reaches 1.24V.
The typical soft-start capacitance ranges from 22nF to
220nF. A 100nF capacitor is usually sufficient for most
of the applications.
SS
C
I
SS
SS
junction
IN
–
voltage is higher than UVLO value, a 4µA
T
SS
temperature
exceeds
AAT1187A
AVDD
FB
voltage. The
September 2009
voltage, the
160
C ,
SS.