NCP5331 datasheet(18/38 Pages) ONSEMI

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NCP5331

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Figure 20. Overvoltage Occurs with UVLO Enabled

NOTE:

Using the lower MOSFETs to prevent overvoltage

is not adequate if the MOSFETs are turned OFF at

the UVLO threshold - VCORE reaches 4.0 V within

100

µs.

Figure 21. Overvoltage Occurs with UVLO Disabled

NOTE:

Even if the lower MOSFETs remain ON after

UVLO, there is not enough gate drive voltage to

prevent VCORE from reaching 4.0 V.

Overvoltage Protection

The NCP5331 provides a comprehensive level of

overvoltage protection. Overvoltage protection (OVP)

addresses the following five cases (in decreasing level of

difficulty):

1. Normal operation, upper MOSFET shorts

2. Upper MOSFET shorted, turn on the ATX power

3. Normal operation, open the voltage feedback signal

4. Normal operation, ground the voltage feedback

signal

5. Open the voltage feedback signal, apply ATX power

By far the most difficult overvoltage scenario is when the

upper MOSFET shorts during normal operation. The energy

stored in the output filters of both the ATX supply and the

dc/dc converter must be dissipated very quickly or an

overvoltage condition will occur. When the upper MOSFET

shorts, VCORE rises and the error amplifier, due to the closed

loop control, will within approximately 400 ns, command

the upper MOSFETs (those that aren’t shorted) to turn OFF

and all the lower MOSFETs to turn ON. This will cause two

things to occur: VCORE will stop increasing, and a very high

current will be drawn from the ATX supply. The current

limit in the ATX supply should become active and the input

voltage to the converter will be removed. Now, when the

input voltage drops below the NCP5331’s UVLO threshold

the lower MOSFETs will be turned OFF. At this point, a fair

amount of the energy in the system will have been

dissipated, however, the converter’s output voltage will

begin to rise again as shown in Figure 20. Even if the lower

MOSFETs are not turned OFF at the UVLO threshold, as

VIN decays, adequate gate drive voltage will not exist to

fully enhance the devices and the CPU may be damaged.

This case is shown in Figure 21.

The NCP5331 avoids the problems with UVLO and the

gate drive voltage. When VCORE exceeds 2.05 V, the

NCP5331 will activate an external crowbar MOSFET via

the CBOUT pin. This additional MOSFET will clamp VCORE

and dissipate the remainder of the energy in the system. The

CBOUT circuitry is powered by 5 VSB and is not disabled

during UVLO. Also, the CBOUT pin will always have

adequate gate drive to enhance the lower MOSFET. The

OVP circuits in the NCP5331 are not effected when the ATX

supply current limits and VIN is removed. Figure 22 and

Figure 23 document successful operation of the CBOUT

circuitry when an upper MOSFET is shorted during normal

operation with 0 A and 45 A loading.

The second most difficult overvoltage scenario is when an

upper MOSFET is shorted and the ATX power is applied. In

this case, VCORE is equal to VIN due to the shorted upper

MOSFET. When VIN reaches the maximum rating for the

CPU (2.2 V) adequate gate drive voltage is not available to

enhance the lower MOSFETs or crowbar device enough to

protect the CPU. A typical “Logic Level” MOSFET will

conduct only 100-300

µA for a gate drive of 2.0-2.5 V

(RDS(on) = 6 kΩ to 25 kΩ). The RDS(on) of the crowbar

device must be lower than 15 m

Ω during startup to prevent

damage to the CPU. The NCP5331 avoids this problem by

taking advantage of the 5 VSB voltage from the ATX supply.

If VIN is less than 5 VSB, then 5 V will be used to enhance

the crowbar device. Most modern MOSFETs will be less

than 10 m

Ω for a VGS greater than 4.5 V. Figure 24 shows

the NCP5331 preventing VCORE from exceeding 2.0 V with

a shorted upper MOSFET during startup.

If the voltage feedback signal (COREFB+) is broken, a

high value internal pull-up resistor will cause VFFB (and

VFB) to float higher in voltage. As VFFB (and VFB) are

pulled higher, the error amplifier will “think” VCORE is too

high and command a lower and lower duty cycle until

VCORE is driven to 0 V. Without the internal pull-up resistor

the error amplifier would command 100% duty cycle and

VCORE would be driven very high, damaging the CPU.

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