NCP1207A Просмотр технического описания (PDF) - ON Semiconductor

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NCP1207A

PWM Current-Mode Controller for Free Running Quasi-Resonant Operation

ON Semiconductor 

VCC

12 V

10 V

5.3 V

NCP1207A, NCP1207B

REGULATION

OCCURS HERE

LATCHOFF

PHASE

DRV

INTERNAL

FAULT FLAG

DRIVER

PULSES

TIME

TIME

If the fault is relaxed during the VCC

natural fall down sequence, the IC

automatically resumes.

If the fault still persists when VCC

reached VCCON, then the controller

cuts everything off until recovery.

STARTUP PHASE

FAULT IS

RELAXED

FAULT OCCURS HERE

TIME

Figure 24.

Soft-Start

The NCP1207A/B features an internal 1 ms soft-start to

soften the constraints occurring in the power supply during

startup. It is activated during the power on sequence. As

soon as VCC reaches VCCOFF , the peak current is gradually

increased from nearly zero up to the maximum clamping

level (e.g. 1.0 V). The soft-start is also activated during the

overcurrent burst (OCP) sequence. Every restart attempt is

followed by a soft-start activation. Generally speaking, the

soft-start will be activated when VCC ramps up either from

zero (fresh power-on sequence) or 5.3 V, the latchoff

voltage occurring during OCP.

corresponding IC consumption, including the MOSFET

drive, establishes at 1.8 mA (e.g. with an 11 nC MOSFET),

we can calculate the required capacitor using the following

formula:

Dt

+

DV @

i

C,

with

DV

=

2.0ĂV.

Then

for

a

wanted

Dt of 10 ms, C equals 9.0 mF or 22 mF for a standard value.

When an overload condition occurs, the IC blocks its

internal circuitry and its consumption drops to 330ĂmA

typical. This happens at VCC = 10ĂV and it remains stuck

until VCC reaches 5.3ĂV: we are in latchoff phase. Again,

using the calculated 22ĂmF and 330ĂmA current consumption,

this latchoff phase lasts: 313Ăms.

Calculating the VCC Capacitor

As the above section describes, the fall down sequence

depends upon the VCC level: how long does it take for the

VCC line to go from 12 V to 10 V? The required time depends

on the startup sequence of your system, i.e. when you first

apply the power to the IC. The corresponding transient fault

duration due to the output capacitor charging must be less

than the time needed to discharge from 12ĂV to 10ĂV,

otherwise the supply will not properly start. The test consists

in either simulating or measuring in the lab how much time

the system takes to reach the regulation at full load. Let's

suppose that this time corresponds to 6.0Ăms. Therefore a

VCC fall time of 10Ăms could be well appropriated in order

to not trigger the overload detection circuitry. If the

HV Pin Recommended Protection

When the user unplugs a power supply built with a QR

controller such as the NCP1207A/B, one instance can occur:

A negative ringing can take place on pin8 due to a

resonance between the primary inductance and the bulk

capacitor. As any CMOS device, the NCP1207A/B is

sensitive to negative voltages that could appear on it's pins

and could create an internal latch-up condition.

For this reason, we recommend adding a resistor between

the bulk capacitor and the VCC pin.

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