ISL6740IBZ datasheet(21/30 Pages) RENESAS

21 / 30 page

ISL6740, ISL6741

FN9111 Rev.7.00

Page 21 of 30

Jul 7, 2017

To obtain the proper value for RTD, Equation 3 on page 14 is

used. Since there is a 10ns propagation delay in the oscillator

circuit, it must be included in the calculation. The value of RTD

selected is 8.06kΩ.

A similar procedure is used to determine the value of RTC using

Equation 2 on page 14. The value of RTC selected is the series

combination of 17.4kΩ and 1.27kΩ. See section “Overcurrent

Component Selection” on page 21 for further explanation.

Output Filter Design

The output filter inductor and capacitor selection is simple and

straightforward. Under steady state operating conditions the

voltage across the inductor is very small due to the large duty

cycle. Voltage is applied across the inductor only during the

switch transition time, about 45ns in this application. Ignoring

the voltage drop across the SR FETs, the voltage across the

inductor during the ON time with VIN = 48V is:

Where:

VL is the inductor voltage

VS is the voltage across the secondary winding

VOUT is the output voltage

If we allow a current ramp,

I, of 5% of the rated output current,

the minimum inductance required is:

An inductor value of 1.4µH, rated for 18A was selected.

With a maximum input voltage of 53V, the maximum output

voltage is about 13V. The closest higher voltage rated capacitor

is 16V. Under steady state operating conditions the ripple current

in the capacitor is small, so it would seem appropriate to have a

low ripple current rated capacitor. However, a high rated ripple

current capacitor was selected based on the nature of the

intended load, multiple buck regulators. To minimize the output

impedance of the filter, a Sanyo OSCON 16SH150M capacitor in

parallel with a 22µF ceramic capacitor were selected.

Overcurrent Component Selection

There are two circuit areas to consider when selecting the

components for overcurrent protection, current limit and

short-circuit shutdown. The current limit threshold is fixed at 0.6V

while the short-circuit threshold is set to a fraction of the duty

cycle the designer wishes to define as a short-circuit.

The current level that corresponds to the overcurrent threshold

must be chosen to allow for the dynamic behavior of an open

loop converter. In particular, the low inductor ripple current under

steady state operation increases significantly as the duty cycle

decreases.

Figures 14 and 15 show the behavior of the inductor ripple under

steady state and overcurrent conditions. In this example, the

peak current limit is set at 11A. The peak current limit causes

the duty cycle to decrease resulting in a reduction of the average

current through the inductor. The implication is that the converter

cannot supply the same output current in current limit that it can

supply under steady state conditions. The peak current limit

setpoint must take this behavior into consideration. A 3.32Ω

current sense resistor was selected for the rectified secondary of

current transformer T2, corresponding to a peak current limit

setpoint of 16.5A.

The short-circuit protection involves setting a voltage between 0

and 2V on the SCSET pin. The applied voltage divided by 2 is the

percent of maximum duty cycle that corresponds to a

short-circuit when the peak current limit is active. A divider from

RTC to ground provides an easy method to achieve this. The

divider between RTC and GND formed by R13 and R15

determines the percent of maximum duty cycle that corresponds

to a short-circuit. The divider ratio formed by R13 and R15 is:

Therefore, the duty cycle that corresponds to a short-circuit is

6.8% of D max (97.9%), or ~6.6%.

VS VOUT

–

VIN NS 1D

–





2NP

------------------------------------------------ 250



(EQ. 22)

VL TON



I

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



0.25 2.08



0.5

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

1.04

H

(EQ. 23)

FIGURE 14. STEADY STATE SECONDARY WINDING VOLTAGE AND

INDUCTOR CURRENT

0.9950

0.9960

0.9970

0.9980

0.9990

1.000

TIME (ms)

V (L1:1)

I (L1)

FIGURE 15. SECONDARY WINDING VOLTAGE AND INDUCTOR

CURRENT DURING CURRENT LIMIT OPERATION

0.986

0.988

0.990

0.992

0.994

1.000

TIME (ms)

0.996

0.998

V (L1:1)

I (L1)

R15

R13 R15

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

1.27k

1.27k 17.4k

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

0.068

(EQ. 24)

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