AP2014/A
SYNCHRONOUS PWM CONTROLLER
AP2014/A Rev. 5 12 of 14 FEBRUARY 2009
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Functional Descriptions
Introduction
The AP2014 is a fixed frequency, voltage mode synchronous controller and consists of a precision reference voltage, an error amplifier,
an internal oscillator, a PWM comparator, 0.5A peak gate driver, soft-start and shutdown circuits (see Block Diagram).
The output voltage of the synchronous converter is set and controlled by the output of the error amplifier; this is the amplified error signal
from the sensed output voltage and the reference voltage.
This voltage is compared to a fixed frequency linear sawtooth ramp and generates fixed frequency pulses of variable duty-cycle, which
drives the two N-channel external MOSFETs.The timing of the IC is provided through an internal oscillator circuit which uses on-chip
capacitor to set the oscillation frequency to 200 KHz (400 KHz for “A” version).
Soft-Start
The AP2014 has a programmable soft-start to control the output voltage rise and limit the current surge at the start-up. To ensure correct
start-up, the soft-start sequence initiates when the V
C
and V
CC
rise above their threshold (3.3V and 4.2V respectively) and generates the
Power On Reset (POR) signal. Soft-start function operates by sourcing an internal current to charge an external capacitor to about 3V.
Initially, the soft-start function clamps the E/A’s output of the PWM converter. As the charging voltage of the external capacitor ramps up,
the PWM signals increase from zero to the point the feedback loop takes control.
Short-Circuit Protection
The outputs are protected against the short circuit. The AP2014 protects the circuit for shorted output by sensing the output voltage
(through the external resistor divider). The AP2014 shuts down the PWM signals, when the output voltage drops below 0.6V (0.4V for
AP2014A).
The AP2014 also protects the output from over-voltaging when the control FET is shorted. This is done by turning on the sync FET with
the maximum duty cycle.
Under-Voltage Lockout
The under-voltage lockout circuit assures that the MOSFET driver outputs remain in the off state whenever the supply voltage drops
below set parameters. Lockout occurs if V
C
and V
CC
fall below 3.3V and 4.2V respectively. Normal operation resumes once V
C
and V
CC
rise above the set values.
IC Quiescent Power Dissipation
Power dissipation for IC controller is a function of applied voltage, gate driver loads and switching frequency. The IC's maximum power
dissipation occurs when the IC operating with single 12V supply voltage (Vcc=12V and Vc
≅24V) at 400KHz switching frequency and
maximum gate loads.
Page 8 shows voltage vs. current, when the gate drivers loaded with 1500pF capacitors. The IC's power dissipation results in an
excessive temperature rise. This should be considered when using AP2014A for such application.
(18 pages)
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