ISL6398 Datasheet, PDF(27/57 Page) Intersil Corporation – Programmable soft-start rate and DVID rate

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ISL6398 Datasheet, PDF (27/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

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ISL6398

VOUT

VSEN_OVP

VOUT

VCC

VSEN_OVP

DAC + OVP

A. INCREASED OVP

B. REDUCED OVP

FIGURE 18. EXTERNAL PROGRAMMABLE OVP

Furthermore the regulation loop (VSEN pin) and the OVP sense

(VSEN_OVP) are separated paths, the OVP level can be

programmed higher or lower than the target, as in Figure 18. The

OVP level cannot be scaled too close to DAC to ensure that the

OVP is not triggered during transient response and start-up.

In addition, the ISL6398 also provides early OVP warning; when it

is triggered, it asserts STATUS_WORD (79h Upper Byte, Bit7) and

SM_PMALERT#. It however does not shutdown the system,

assert STATUS_BYTE, or pull VR_RDY low. Disregard this if both

STATUS_WORD and SM_PMALERT# are not used or disable OVP

Warning as needed via DFh.

Undervoltage Protection

When the output voltage drops below a level programmed by

PMBus (E1[3:0]), the VR_RDY is pulled low. The controller can

respond to UVP with two different options programmed by

PMBus (E1[6]): 1) acts as a OCP event, hiccup the output with

9ms duration and pull VR_RDY low; or 2) acts like a PGOOD, pull

VR_RDY low, monitor only. To avoid faulty triggering at

transient/DVID events, the UVP delay is programmable by

E1[5:4]. Furthermore, the UVP is not enabled during soft-start

and also can be disabled by DF[5]. The ISL6398 also provides

early UVP warning; when it is triggered, it asserts STATUS_WORD

(79h Upper Byte, Bit7) and SM_PMALERT#. It however does not

shutdown the system, assert STATUS_BYTE or pull VR_RDY low.

Disregard this if both STATUS_WORD and SM_PMALERT# are not

used or disable UVP Warning as needed via DFh.

Overcurrent Protection

The ISL6398 has two levels of overcurrent protection. Each phase

is protected from a sustained overcurrent condition by limiting its

peak current, while the combined phase currents are protected on

an instantaneous basis.

For the individual channel overcurrent protection, the ISL6398

continuously compares the sensed peak current (~50ns filter)

signal of each channel with the reference current (ICL, typically

125ÂµA, programmable via F4[5:3] and F3[2:0]). If one channel

current exceeds the reference current, the ISL6398 will pull PWM

signal of this channel to low for the rest of the switching cycle.

This PWM signal can be turned on next cycle if the sensed

channel current is less than the reference current. The peak

current limit of individual channels will only use cycle-by-cycle

current limiting and will not trigger the converter to shut down.

In instantaneous protection mode, the ISL6398 utilizes the

sensed average current IAVG to detect an overcurrent condition.

Refer to âCurrent Sensingâ on page 18 for more details on how

the average current is measured. The average current is

continually compared with a reference current (typically 100ÂµA,

programmable via F2[2:0]), as shown in Figure 17. Once the

average current exceeds the reference current, a comparator

triggers the converter to shut down. In addition, the current out of

the IMON pin is equal to the sensed average current IAVG. With a

resistor from IMON to GND, the voltage at IMON will be

proportional to the sensed average current and the resistor value.

The ISL6398 continuously monitors the voltage at the IMON pin. If

the voltage at the IMON pin is higher than 3.0V, a precision

comparator triggers the overcurrent shutdown. Since the internal

current comparator has wider tolerance than the voltage

comparator, the IMON voltage comparator is the preferred one for

OCP trip. Therefore, the resistor between IMON and GND can be

scaled such that the overcurrent protection threshold is tripping

lower than 100ÂµA. For example, the overcurrent threshold for the

sensed average current IAVG can be set to 95ÂµA by using a 31.5kï

resistor from IMON to GND. Thus, the internal 100ÂµA comparator

might only be triggered at its lower corner. However, IMON OCP trip

should NOT be too far away from 125ÂµA, which is used for

cycle-by-cycle protection and inductor saturation.

OUTPUT CURRENT

OUTPUT VOLTAGE

2ms/DIV

FIGURE 19. OVERCURRENT BEHAVIOR IN HICCUP MODE

FSW = 500kHz

At the beginning of overcurrent shutdown, the controller places all

PWM signals in a high-impedance state, commanding the Intersil

MOSFET driver ICs to turn off both upper and lower MOSFETs. The

system remains in this state a period of 9ms. If the controller is still

enabled at the end of this wait period, it will attempt a soft-start. If

the fault remains, the trip-retry cycles will continue indefinitely

(as shown in Figure 19) until either controller is disabled or the fault

is cleared. Note that the energy delivered during trip-retry cycling is

much less than during full-load operation, so there is no thermal

hazard during this kind of operation.

Thermal Monitoring (VR_HOT#)

VR_HOT# indicates the temperature status of the voltage

regulator. VR_HOT# is an open-drain output, and an external

pull-up resistor is required. This signal is valid only after the

controller is enabled.

The VR_HOT# signal can be used to inform the system that the

temperature of the voltage regulator is too high.

Submit Document Feedback 27

FN8575.1

August 13, 2015

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