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NCV8851-1

Automotive Grade Synchronous Buck Controller

ON Semiconductor 

NCV8851−1

VIN

REN

EN

125 kΩ

DZEN2

22 V

Internal

Enable

DZEN1

5.4 V

Figure 20. Enable Pin Equivalent Structure

The EN pin can be tied to VIN in order to enable the part.

If EN is above 22 V, DZEN2 will be conducting, as well as

DZEN1. The current to DZEN1 is limited by an internal

125 kW resistor. If DZEN2 is conducting, it is recommended

at least 250 mA is pulled through this diode. The resistor REN

must be used if VIN can go above 20 V as follows.

REN(max)

+

VZ

250 mA

Where VZ is the amount of volts where DZEN2 is conducting,

but not yet supplied with 250 mA. For example, setting VZ

to 1 V means REN must be less than 4 kW for DZEN2 to have

at least 250 mA when VIN is at least 23 V; for the range of VIN

between 22 V and 23 V, DZEN2 will be conducting, but not

with the recommended 250 mA current.

UVLO

Undervoltage Lockout (UVLO) is provided to ensure that

unexpected behavior does not occur when VIN_IC is too low

to support the internal rails and power the controller. The IC

will start up when enabled and VIN_IC surpasses the UVLO

threshold and will shutdown when VIN_IC drops below the

UVLO threshold minus the UVLO hysteresis. While VIN is

less than the set point for VOUT, the output will run at max

duty cycle, after soft−start, once VIN_IC surpasses the

UVLO threshold. If EN is high and not tied to VIN, the

output will begin to rise up while in UVLO, if a minimum

output load of 1 kW is not met.

Thermal Shutdown

The NCV8851−1 provides Thermal Shutdown (TSD),

which monitors the die temperature and turns off the top and

bottom gate drivers if an over temperature condition is

detected, for added protection. The internal soft−start

capacitor is also discharged. A normal soft−start will occur

when the die temperature falls below the TSD threshold

minus the TSD hysteresis.

Duty Cycle and Maximum Pulse Width Limits

In steady state dc operation, the duty cycle will stabilize

at an operating point defined by the ratio of the input to the

output voltage. There is a built in minimum off−time which

ensures that the bootstrap supply is charged every cycle,

determining the maximum duty cycle at a given frequency.

The NCV8851−1 can achieve at least a 95% duty cycle

while operating at frequencies up to 200 kHz (89% at up to

500 kHz).

Internal Soft−Start

The NCV8851−1 features an internal soft−start function,

which reduces inrush current and overshoot of the output

voltage. Figures 21 and 22 show a typical soft−start

sequence.

VIN

6VOUT

and

VIN_IC

UVLO

Threshold

Soft−start Time

90%*VOUT

VOUT

t

Soft−start Delay

10%*VOUT

Figure 21. Normal Start−up

Soft−start is achieved by ramping up the internal soft−start

voltage (VSS), which is applied to the non−inverting input of

the voltage error amplifier, effectively limiting the slew rate

of VOUT rising. This ramp is generated by charging an

internal soft−start capacitor based on the internal oscillator,

VSW

t

Figure 22. Switch−node in Soft−Start

causing the soft−start time to be inversely related to the

frequency set by ROSC. The internal soft−start capacitor is

discharged when the part is disabled, enters TSD or enters

UVLO, ensuring a proper start−up when the part is

re−enabled, leaves TSD or leaves UVLO.

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