BL8501
The following description refer to the functional block diagram:
Error Amplifier
The error amplifier is a high-gain differential amplifier used to regulate the converter output
voltage. The amplifier generates an error signal, which is fed to the PWM comparator. The
error signal is generated when a sample of the output voltage is compared to the internal
reference and the difference is amplified. The output sample is obtained from
V
FB
, which is
obtained from an external resistive divider connected between output voltage and GND.
PWM Comparator
The PWM comparator generates a series of pulses, which is obtained when the error signal is
compared to the oscillator clock. And the series of pulses are used to control the state of the
power switch by turning the driver on or off.
PWM/VFM controller
With a PWM/VFM alternative circuit, when the load current is small, the operation is
automatically switching into the VFM oscillator from PWM oscillator. Therefore, the efficiency
at small load current is improved.
Oscillator
The oscillator circuit provides a 500kHz clock, to set the converter operating frequency, and a
timing ramp of slope compensation. The clock waveform is a pulse, a few hundred
nanoseconds in duration.
Protection circuit
If the term of maximum duty cycle keeps on a certain time, the embedded protection circuit
works. The protection circuit is Reset-type protection circuit, and it works to restart the
operation with soft-start and repeat this operation until maximum duty cycle condition is
released. When the cause of large load current or something else is removed, the operation
is automatically released and returns to normal operation.
Soft-Start
The Soft-Start circuit causes the output voltage to increase to the regulation point at a
controlled rate of rise. The voltage on the charging soft-start capacitor gradually raises the
reference voltage of the error amplifier to clamp the error signal.
UVLO
UVLO function works when the input voltage is equal or less than UVLO threshold, it makes
this IC be standby and suppresses the consumption current and avoids an unstable
operation.
Enable (CE)
Logic low on CE puts the BL8501 in standby state. In standby mode, the output power switch,
voltage reference, and other functions are shut off, the supply current is reduced to 1 A
maximum.
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Pages
8/8/2006
BELLING [ SHANGHAI BELLING CO., LTD. ]
