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NCP1521 Datasheet(PDF) 9 Page - ON Semiconductor

Part No. NCP1521
Description  1.5 MHz, 600 mA, High−Efficiency, Low Quiescent Current, Adjustable Output Voltage Step−Down Converter
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1521 Datasheet(HTML) 9 Page - ON Semiconductor

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Cycle−by−Cycle Current Limitation
From the block diagram (Figure 4), an ILIM comparator
is used to realize cycle−by−cycle current limit protection.
The comparator compares the LX pin voltage with the
reference voltage, which is biased by a constant current. If
the inductor current reaches the limit, the ILIM comparator
detects the LX voltage falling below the reference voltage
and releases the signal to turn off the switch Q1. The
cycle−by−cycle current limit is set at 1200 mA (nom).
Short Circuit Protection
When the output is shorted to ground, the device limits
the inductor current. The duty−cycle is minimum and the
consumption on the input line is 300 mA (Typ). When the
short circuit condition is removed, the device returns to the
normal mode of operation.
The NCP1521 uses soft−start (300
ms Typ) to limit the
inrush current when the device is initially enabled.
Soft−start is implemented by gradually increasing the
reference voltage until it reaches the full reference voltage.
During startup, a pulsed current source charges the internal
soft−start capacitor to provide gradually increasing
reference voltage. When the voltage across the capacitor
ramps up to the nominal reference voltage, the pulsed
current source will be switched off and the reference
voltage will switch to the regular reference voltage.
Shutdown Mode
When the EN pin has a voltage applied of less than 0.4 V,
the NCP1521 will be disabled. In shutdown mode, the
internal reference, oscillator and most of the control
circuitries are turned off. Therefore, the typical current
consumption will be 0.3
mA (typical value). Applying a
voltage above 1.2 V to EN pin will enable the device for
normal operation. The typical threshold is around 0.7 V.
The device will go through soft−start to normal operation,
however, the EN pin should be tied low while the input
voltage on VIN pin is rising up.
Thermal Shutdown
Internal Thermal Shutdown circuitry is provided to
protect the integrated circuit in the event that the maximum
temperature exceeds 160
_C, the device shuts down. In this
mode switch Q1 and Q2 and the control circuits are all
turned off. The device restarts in soft−start after the
temperature drops below 135
_C. This feature is provided
to prevent catastrophic failures from accidental device
overheating, and it is not intended as a substitute for proper
Low Dropout Operation
The NCP1521 offers a low input to output voltage
difference. The NCP1521 can operate at 100% duty cycle.
In this mode the PMOS (Q1) switches completely on.
The minimum input voltage to maintain regulation can
be calculated as:
VIN(min) + VOUT(max)
(eq. 1)
VOUT: Output Voltage (Volts)
IOUT: Max Output Current
RDS(on): P−Channel Switch RDS(on)
RINDUCTOR: Inductor Resistance (DCR)

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