ISL8272MEVAL2Z datasheet(20/59 Pages) RENESAS

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ISL8272M

FN8670 Rev.5.00

Page 20 of 59

Nov 8, 2017

Output Overvoltage Protection

The ISL8272M offers an internal output overvoltage protection

circuit that can protect sensitive load circuitry from being

subjected to a voltage higher than its prescribed limits. A

hardware comparator compares the actual output voltage (seen

at the VSENP and VSENN pins) to a threshold set to 15% higher

than the target output voltage (the default setting). The fault

threshold can be programmed to a desired level with the PMBus

command VOUT_OV_FAULT_LIMIT. If the VSENP voltage exceeds

this threshold, the module will initiate an immediate shutdown

without retry. A 70ms continuous retry can be enabled with the

PMBus command VOUT_OV_FAULT_RESPONSE. Note that fault

retry is not supported in the current sharing configuration.

Internal to the module, two 22Ω resistors are populated from

VOUT to VSENP and SGND to VSENN to protect from overvoltage

conditions in case of open at voltage sensing pins and

differential remote sense traces due to assembly error. As long

as differential remote sense traces have low resistance, VOUT

regulation accuracy is not sacrificed.

Output Prebias Protection

An output prebias condition exists when an externally applied

voltage is present on a power supply’s output before the power

supply’s control IC is enabled. Certain applications require that

the converter not be allowed to sink current during start-up if a

prebias condition exists at the output. The ISL8272M provides

prebias protection by sampling the output voltage prior to

initiating an output ramp.

If a prebias voltage lower than the target voltage exists after the

pre-configured delay period has expired, the target voltage is set

to match the existing prebias voltage and both drivers are

enabled. The output voltage is then ramped to the final

regulation value at the pre-configured ramp rate.

The actual time the output takes to ramp from the prebias

voltage to the target voltage varies, depending on the prebias

voltage; however, the total time elapsed from when the delay

period expires to when the output reaches its target value will

match the pre-configured ramp time (see Figure 26).

If a prebias voltage is higher than the target voltage after the pre-

configured delay period has expired, the target voltage is set to

match the existing prebias voltage and both drivers are enabled

with a PWM duty cycle that would ideally create the prebias

voltage.

Once the preconfigured soft-start ramp period has expired, the

PG pin is asserted (assuming the prebias voltage is not higher

than the overvoltage limit). The PWM then adjusts its duty cycle

to match the original target voltage and the output ramps down

to the pre-configured output voltage.

If a prebias voltage is higher than the overvoltage limit, the

device does not initiate a turn-on sequence and declares an

overvoltage fault condition. The device then responds based on

the output overvoltage fault response setting programmed with

the PMBus command VOUT_OV_FAULT_RESPONSE.

Output Overcurrent Protection

The ISL8272M can protect the power supply from damage if the

output is shorted to ground or if an overload condition is imposed

on the output. Average output overcurrent fault threshold can be

programmed with the PMBus command IOUT_OC_FAULT_LIMIT.

The module automatically programs the peak inductor current

fault threshold by calculating the inductor ripple current from the

input voltage, switching frequency, and the VOUT_COMMAND.

When the peak inductor current crosses the peak inductor

current fault threshold for three successive switching cycles it will

initiate an immediate shutdown.

The default response from an overcurrent fault is an immediate

shutdown without retry. A 70ms continuous retry can be enabled

with the PMBus command MFR_IOUT_OC_FAULT_RESPONSE. It

is required to enable the output undervoltage fault retry with the

PMBus command VOUT_UV_FAULT_RESPONSE command

simultaneously if the overcurrent fault retry is enabled. Note that

fault retry is not supported in the current sharing configuration.

82.5

2Fh

90.9

30h

100

31h

110

32h

121

33h

133

34h

147

35h

162

36h

178

37h

TABLE 8. SMBus ADDRESS RESISTOR SELECTION (Continued)

RSA (kΩ)SMBus ADDRESS

FIGURE 26. OUTPUT RESPONSES TO PREBIAS VOLTAGES

DESIRED OUTPUT

VOLTAGE

PREBIAS

VOLTAGE

VOUT

TIME

TON-

DELAY

TON-

RISE

DESIRED OUTPUT

VOLTAGE

PREBIAS

VOLTAGE

VOUT

TIME

VPREBIAS < VTARGET

VPREBIAS > VTARGET

TON-

RISE

TON-

DELAY

ISL8272MEVAL2Z Datasheet(PDF) 20 Page - Renesas Technology Corp

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