ISL6740IBZ datasheet(14/30 Pages) RENESAS

14 / 30 page

ISL6740, ISL6741

FN9111 Rev.7.00

Page 14 of 30

Jul 7, 2017

OTS - The noninverting input to the over-temperature shutdown

comparator. The signal input at this pin is compared to an

internal threshold of VREF/2. If the voltage at this pin exceeds the

threshold, the Fault signal is asserted and the outputs are

disabled until the condition clears. There is a nominal 25µA

switched current source used for hysteresis. The amount of

hysteresis is adjustable by varying the source impedance of the

signal into this pin.

OTS may be used to monitor parameters other than temperature,

such as voltage. Any signal for which a high out-of-bounds

monitor is desired may utilize the OTS comparator.

FAULT - The Fault signal is asserted high whenever the outputs,

OUTA and OUTB, are disabled. This occurs during an

over-temperature fault, an input UV fault, a VREF UV fault, or

during an overcurrent (ISL6740) or short-circuit shutdown fault.

Fault can be used to disable synchronous rectifiers whenever the

outputs are disabled.

Fault is a three-state output and is high impedance during the

soft-start cycle. Adding a pull-up resistor to VREF or a pull-down

resistor to ground determines the state of Fault during soft-start.

This feature allows the designer to use the Fault signal to enable

or disable output synchronous rectifiers during soft-start.

UV - Undervoltage monitor input pin. A resistor divider between

the input source voltage and GND sets the undervoltage lock out

threshold. The signal is compared to an internal 1.00V reference

to detect an undervoltage or inhibit condition.

CS - This is the input to the current sense comparator(s). The IC

has the PWM comparator for peak current mode control

(ISL6741) and an overcurrent protection comparator. The

overcurrent comparator threshold is set at 0.600V nominal.

The CS pin is shorted to GND at the end of each switching cycle.

Depending on the current sensing source impedance, a series

input resistor may be required due to the delay between the

internal clock and the external power switch. This delay may

allow an overlap such that the CS signal may be discharged while

the current signal is still active. If the current sense source is low

impedance, it will cause increased power dissipation.

ISL6740 - Exceeding the overcurrent threshold will start a

delayed shutdown sequence. Once an overcurrent condition is

detected, the soft-start charge current source is disabled. The

soft-start capacitor begins discharging through a 25µA current

source, and if it discharges to less than 4.25V (sustained

overcurrent threshold), a shutdown condition occurs and the

OUTA and OUTB outputs are forced low. When the soft-start

voltage reaches 0.27V (reset threshold) a soft-start cycle begins.

An overcurrent condition must be absent for 50µs before the

delayed shutdown control resets. If the overcurrent condition

ceases, and an additional 50µs period elapses before the

shutdown threshold is reached, no shutdown occurs. The SS

charging current is re-enabled and the soft-start voltage is

allowed to recover.

ISL6741 - The ISL6741 current mode controller does not shut

down due to an overcurrent condition. The pulse-by-pulse current

limit characteristic of peak current mode control limits the

output current to acceptable levels.

GND - Reference and power ground for all functions on this

device. Due to high peak currents and high frequency operation,

a low impedance layout is necessary. Ground planes and short

traces are highly recommended.

OUTA and OUTB - Alternate half cycle output stages. Each output

is capable of 0.5A peak currents for driving logic level power

MOSFETs or MOSFET drivers. Each output provides very low

impedance to overshoot and undershoot.

VREF - The 5.00V reference voltage output. +1%/-2% tolerance

over line, load and operating temperature. Bypass to GND with a

0.047µF to 2.2µF ceramic capacitor. Capacitors outside of this

range may cause oscillation.

SS - Connect the soft-start timing capacitor between this pin and

GND to control the duration of soft-start. The value of the

capacitor determines the rate of increase of the duty cycle during

start-up, controls the overcurrent shutdown delay (ISL6740), and

the overcurrent and short-circuit hiccup restart period.

SCSET - Sets the duty cycle threshold that corresponds to a

short-circuit condition. A resistive divider between RTC and GND

or RTD and GND, or a voltage between 0V and 2V may be used to

adjust the SCSET threshold. If using a resistor divider from either

RTC or RTD, the impedance to GND affects the oscillator timing

and should be considered when determining the oscillator timing

components. Connecting SCSET to GND disables short-circuit

shutdown and hiccup.

Functional Description

Features

The ISL6740, ISL6741 PWMs are an excellent choice for low cost

bridge and push-pull topologies for applications requiring

accurate duty cycle and dead time control. With its many

protection and control features, a highly flexible design with

minimal external components is possible. Among its many

features are current mode control (ISL6741), adjustable

soft-start, overcurrent protection, thermal protection,

bidirectional synchronization, fault indication and adjustable

frequency.

Oscillator

The ISL6740, ISL6741 have an oscillator with a programmable

frequency range to 2MHz, which can be programmed with two

resistors and capacitor. The use of three timing elements, RTC,

RTD, and CT allow great flexibility and precision when setting the

oscillator frequency.

The switching period may be considered the sum of the timing

capacitor charge and discharge durations. The charge duration is

determined by RTC and CT. The discharge duration is determined

by RTD and CT.

tC 0.5 RTC





(EQ. 2)

tD 0.02 RTD





(EQ. 3)

tSW

tC tD

fSW

----------

(EQ. 4)

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