ISL8270MEVAL1Z datasheet(18/56 Pages) RENESAS

18 / 56 page

ISL8270M

FN8635 Rev.4.00

Page 18 of 56

Aug 17, 2017

Output Overvoltage Protection

The ISL8270M offers an internal output overvoltage protection

circuit that can be used to protect sensitive load circuitry from

being subjected to a voltage higher than its prescribed limits. A

hardware comparator is used to compare the actual output

voltage (seen at the VSEN+, VSEN- pins) to a threshold set to 15%

higher than the target output voltage (the default setting). Fault

threshold can be programmed to a desired level with the PMBus

command VOUT_OV_FAULT_LIMIT. If the VSEN+ voltage exceeds

this threshold, the module will initiate an immediate shutdown

without retry. Retry settings can be programmed with the PMBus

command VOUT_OV_FAULT_RESPONSE.

Internal to the module, two 100Ω resistors are populated from

VOUT to VSEN+ and SGND to VSEN- to protect from overvoltage

conditions in case of open at VSENSE 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 ISL8270M 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

preconfigured 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 preconfigured 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 and when the output reaches its target value will

match the preconfigured ramp time (see Figure 27).

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

preconfigured 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.

When 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 preconfigured 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

PMBus command VOUT_OV_FAULT_RESPONSE.

Output Overcurrent Protection

The ISL8270M can protect the power supply from damage if the

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

on the output. The average output overcurrent fault threshold can

be programmed with PMBus command IOUT_OC_FAULT_LIMIT.

The module automatically programs the peak inductor current

fault threshold, by calculating the inductor ripple current by

reading real-time input voltage, switching frequency, and

VOUT_COMMAND. When the peak inductor current crosses the

peak inductor current fault threshold for five successive cycle

modules it will initiate an immediate shutdown.

The default response from an overcurrent fault is an immediate

shutdown without retry. Retry settings can be programmed with

the PMBus command MFR_IOUT_OC_FAULT_RESPONSE.

Thermal Overload Protection

The ISL8270M includes a thermal sensor that continuously

measures the internal temperature of the module and shuts

down the controller when the temperature exceeds the preset

limit. The default temperature limit is set to +125°C in the

factory, but can be changed with the PMBus command

OT_FAULT_LIMIT.

The default response from an over-temperature fault is an

immediate shutdown without retry. Retry settings can be

programmed with the PMBus command OT_FAULT_RESPONSE.

If the user has configured the module to retry, the controller

waits the preset delay period (if configured to do so) and then

checks the module temperature. If the temperature has dropped

below a threshold that is approximately +15°C lower than the

82.5

2Fh

90.9

30h

100

31h

TABLE 7. SMBus ADDRESS RESISTOR SELECTION (Continued)

RSA

(kΩ)

SMBus

ADDRESS

FIGURE 27. 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

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