NCP1529

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11

Soft−Start

The NCP1529 uses soft−start to limit the inrush current

when the device is initially powered up or 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.

Cycle−by−cycle Current Limitation

realize cycle−by−cycle current limit protection. The

comparator compares the SW pin voltage with the reference

voltage, which is biased by a constant current. If the inductor

SW 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 1600 mA (nom).

Low Dropout Operation

The NCP1529 offers a low input to output voltage

difference. The NCP1529 can operate at 100% duty cycle.

In this mode the PMOS (Q1) remains completely ON. The

minimum input voltage to maintain regulation can be

calculated as:

(eq. 1)

Undervoltage Lockout

NCP1529 enables the DC/DC converter output to begin the

start up sequence (see soft−start section). The UVLO

threshold hysteresis is typically 100 mV.

Shutdown Mode

Forcing this pin to a voltage below 0.4 V will shut down

the IC. 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 DC/DC converter for normal operation. The device will

go through soft−start to normal operation.

Thermal Shutdown

Internal Thermal Shutdown circuitry is provided to

protect the integrated circuit in the event that the maximum

junction Temperature is exceeded. If the junction

temperature exceeds 180

°C, the device shuts down. In this

mode all power transistors and control circuits are turned

off. The device restarts in soft−start after the temperature

drops below 140

°C. This feature is provided to prevent

catastrophic failures from accidental device overheating.

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 550 mA (typ). When the

short circuit condition is removed, the device returns to the

normal mode of operation.

USB or 5 V Rail Powered Applications

For USB or 5 V rail powered applications, NCP1529 is

able to supply voltages up to 3.9 V, 600 mA, operating in

PWM mode only, with high efficiency (Figure 28), low

output voltage ripple and good load regulation results over

all current range (Figure 29).

40

45

50

55

60

65

70

75

80

85

90

95

100

0

200

400

600

800

1000

Figure 28. Efficiency vs. Output Current

25°C

85°C

−40°C

−3.0

−2.5

−2.0

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

200

400

600

800

1000

Figure 29. Load Regulation vs. Output Current

25°C

85°C

−40°C