ISL8117

FN8666 Rev 4.00

Page 17 of 24

Apr 19, 2018

Overcurrent Protection

monitor the current in the converter. The sensed voltage drop is

from the LGATE/OCS pin to ground during the initiation stage

before soft-start. During the initiation stage, a 10.5µA current

source from LGATE/OCS pin creates a voltage drop on ROCSET.

The voltage drop is then read and stored as the OCP comparator

is the value of the current sense resistor connected to the ISEN

If an overcurrent is detected, the upper MOSFET remains off and

the lower MOSFET remains on until the next cycle. As a result, the

converter will skip a pulse. When the overload condition is

removed, the converter will resume normal operation.

If an overcurrent is detected for two consecutive clock cycles, the

IC enters in a Hiccup mode by turning off the gate driver and

entering soft-start. The IC will stay off for 50ms before trying to

restart. The IC will continue to cycle through soft-start until the

overcurrent condition is removed. Hiccup mode is active during

soft-start, so care must be taken to ensure that the peak inductor

current does not exceed the overcurrent threshold during

soft-start.

Because of the nature of this current sensing technique, and to

overcurrent threshold should represent an overload current about

150% to 180% of the maximum operating current. If more

accurate current protection is desired, place a current sense

resistor in series with the lower MOSFET source.

When OCP is triggered, the SS/TRK pin is pulled to ground by an

internal MOSFET for hiccup restart. When configured to track

another voltage rail, the SS/TRK pin rises up much faster than

the internal minimum soft-start ramp. The voltage reference will

then be clamped to the internal minimum soft-start ramp. Thus,

smooth soft-start hiccup is achieved even with the tracking

function.

For applications with large inductor ripple current, it is

into the ISEN pin to less than 6µA which is the OCP comparator

hysteresis. Otherwise, when the load current approaches to the

OCP trip point, the OCP comparator can trip and reset in one

switching cycle. The overcurrent condition cannot last for two

consecutive cycles to force the IC into Hiccup mode. Instead, the

IC will run in a half frequency PWM mode leading to a larger

output ripple.

Overvoltage Protection

The overvoltage set point is set at 121% of the nominal output

voltage set by the feedback resistors. In the case of an

overvoltage event, the IC will attempt to bring the output voltage

back into regulation by keeping the upper MOSFET turned off and

the lower MOSFET turned on. If the overvoltage condition has

been corrected and the output voltage returns to 110% of the

nominal output voltage, both upper and lower MOSFETs will be

turned off until the output voltage drops to the nominal voltage

to start work in normal PWM switching.

For lower control loop bandwidth applications, such as very low

output voltage or very low switching frequency designs, the full

load to no load transient response may be slow to cause an OVP

false trigger. When OVP is triggered, the long LGATE on-time will

create a high negative inductor current leading to a higher than

normal sink in current to the ISEN pin. It is recommended to limit

the ISEN pin sink in current to less than 16µA. Otherwise, a false

OCP hiccup operation may be triggered to cause the output to

shut down.

Over-Temperature Protection

The IC incorporates an over-temperature protection circuit that

shuts the IC down when a die temperature of +160°C is

reached. Normal operation resumes when the die temperature

drops below +145°C through the initiation of a full soft-start

cycle. During OTP shutdown, the IC consumes only 100µA of

current. When the controller is disabled, thermal protection is

inactive; this helps achieve a very low shutdown current of 5µA.

Feedback Loop Compensation

To reduce the number of external components and to simplify the

process of determining compensation components, the

controller is designed with an internally compensated error

amplifier. To make internal compensation possible, several

design measures were taken.

First, the ramp signal applied to the PWM comparator is

proportional to the input voltage provided at the VIN pin. This

keeps the modulator gain constant with varying input voltages.

Next, the load current proportional signal is derived from the

voltage drop across the lower MOSFET during the PWM time

interval and is subtracted from the amplified error signal on the

comparator input. This creates an internal current control loop.

current feedback loop. The following expression estimates the

required value of the current sense resistor depending on the

maximum operating load current and the value of the MOSFET

and hold circuitry is recommended but values down to 2µA and

up to 100µA can be used.

Due to the current loop feedback, the modulator has a single pole

response with -20dB slope at a frequency determined by the load

by using Equation 9.

type of modulator, a Type 2 compensation circuit is usually

sufficient.

Figure 32 on page 18 shows a Type 2 amplifier and its response,

along with the responses of the current mode modulator and the



 I

OC



+

------------------------------------------- k





=

(EQ. 7)

 r

DS ON





30

A

-----------------------------------------------



(EQ. 8)

1

2

 R



---------------------------------

=

(EQ. 9)