NCV8853

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9

Gate Drive

To turn on the P−Channel MOSFET, the gate driver turns

on a current source to ground. A clamp ensures that the gate

drive voltage does not exceed 10 V. When the clamp starts

conducting the current source starts to turn off. To turn off

the external MOSFET, the gate driver turns on a current

100% Duty Cycle Operation

Each cycle, the oscillator allows either a maximum duty

cycle up to 93% or 100% duty cycle operation. The

oscillator does not allow duty cycles between 93% and

100%.

Every cycle, the oscillator determines whether an

off−time is necessary. If so, the oscillator creates a duty cycle

up to 93%. If an off−time is not required, it can be skipped

and 100% duty cycle is allowed for the cycle.

Below are a few examples of what this could look like on

the switching node:

Figure 14. Duty Cycle Timing

A

B

D

1234

C

≤93%

93%

93%

100%

93%

100%

93%

100%

93%

100%

100%

100%

100%

≤93%

≤93%

≤93%

A: Continuous operation. Each period has a duty cycle that

is less than or equal to 93%.

B: One off−time is skipped in period 3, while the minimum

off−time is maintained in periods 1, 2, and 4.

C: An off−time is skipped in period 1 and in period 3, while

the minimum off−time is maintained in periods 2 and 4.

D: Low input voltage causes the IC to regulate at continuous

100% duty cycle (dropout).

EN/SYNC

This pin has three modes. When a dc logic high

(CMOS/TTL compatible) voltage is applied to this pin the

NCV8853 operates at the default frequency. When a dc logic

low voltage is applied to this pin the NCV8853 enters a low

quiescent current sleep mode. When a square wave of at least

40% of the switching frequency is applied to this pin the

switcher operates at the same frequency as the square wave.

If the signal is slower than 40% of the switching frequency,

it will be interpreted as enabling and disabling the part. The

falling edge of the square wave corresponds to the start of the

switching cycle.

Power Good

The power good pin is high when the reference voltage

reaches 90% of its target of 800 mV. The pin should be pulled

up with a 10 k

W resistor to 5 V. The output voltage of the

controller may be used in this case.

Overvoltage Lockout

To protect the IC, if the voltage on the VIN pin the exceeds

below this voltage the part will go through the normal

soft−start procedure.

Undervoltage Lockout

Undervoltage lockout protection is engaged when the

plus hysteresis. Depending on the desired output voltage, it

is possible to engage the short-circuit hiccup mode before

undervoltage lockout occurs.

Soft−Start

To ensure moderate inrush current and reduce output

overshoot, the NCV8853 features a soft start which

periodically adds charge to a capacitor until the final

reference voltage is achieved. When using an external

SYNC signal, charging is based on the switching frequency.

If, for example, the NCV8853 is synchronized to twice the

free running (not synced) frequency, the soft−start will be

half as long.