APW7104 データシートの表示（PDF） - Anpec Electronics
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APW7104 Datasheet PDF : 18 Pages
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APW7104
Function Description
Main Control Loop
The APW7104 is a constant frequency, synchronous rec-
tifier and current-mode switching regulator. In normal
operation, the internal P-channel power MOSFET is
turned on each cycle. The peak inductor current at which
ICMP turn off the P-FET is controlled by the voltage on the
COMP node, which is the output of the error amplifier
(EAMP). An external resistive divider connected between
VOUT and ground allows the EAMP to receive an output
feedback voltage VFB at FB pin. When the load current
increases, it causes a slightly decrease in VFB relative to
the 0.6V reference, which in turn causes the COMP volt-
age to increase until the average inductor current matches
the new load current.
Under-Voltage Lockout
An under-voltage lockout function prevents the device from
operating if the input voltage on VIN is lower than approxi-
mately 1.8V. The device automatically enters the shut-
down mode if the voltage on VIN drops below approxi-
mately 1.8V. This under-voltage lockout function is imple-
mented in order to prevent the malfunctioning of the
converter.
Soft-Start
The APW7104 has a built-in soft-start to control the out-
put voltage rise during start-up. During soft-start, an in-
ternal ramp voltage, connected to the one of the positive
inputs of the error amplifier, raises up to replace the ref-
erence voltage (0.6V typical) until the ramp voltage
reaches the reference voltage. Then, the voltage on FB
regulated at reference voltage.
Enable/Shutdown
Driving RUN to the ground places the APW7104 in shut-
down mode. When in shutdown, the internal power
MOSFETs turn off, all internal circuitry shuts down and
the quiescent supply current reduces to 0.5µA maximum.
Pulse Frequency Modulation Mode (PFM)
The APW7104 is a fixed frequency, peak current mode
PWM step-down converter. At light loads, the APW7104
will automatically enter in pulse frequency mode opera-
tion to reduce the dominant switching losses. In PFM
operation, the inductor current may reach zero or reverse
on each pulse. A zero current comparator turn off the N-
FET, forcing DCM operation at light load. These controls
get very low quiescent current, help to maintain high effi-
ciency over the complete load range.
Slope Compensation and Inductor Peak Current
The APW7104 is a peak current mode PWM step down
converter. To prevent sub-harmonic oscillations, the
APW7104 sense the peak current and add slope com-
pensation to stable the converter. It is accomplished in-
ternally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak cur-
rent for duty cycles > 40%. However, the APW7104 uses a
special scheme that counteracts this compensating
ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
Adaptive Shoot-Through Protection
The gate driver incorporates adaptive shoot-through pro-
tection to high-side and low-side MOSFETs from con-
ducting simultaneously and shorting the input supply.
This is accomplished by ensuring the falling gate has
turned off one MOSFET before the other is allowed to
rise.
During turn-off the low-side MOSFET, the internal LGATE
voltage is monitored until it is below 1.5V threshold, at
which time the UGATE is released to rise after a constant
delay. During turn-off the high-side MOSFET, the UGATE
voltage is also monitored until it is above 1.5V threshold,
at which time the LGATE is released to rise after a con-
stant delay.
Dropout Operation
As the input supply voltage decreases to a value ap-
proaching the output voltage, the duty cycle increases
toward the maximum on time. Further, reduction of the
supply voltage forces the main switch to remain on for
more than one cycle until it reaches 100% duty cycle. The
input voltage minus the voltage drop will determine the
output voltage across the P-FET and the inductor.
Copyright © ANPEC Electronics Corp.
8
Rev. A.6 - Mar., 2012
www.anpec.com.tw
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