ISL6721 datasheet(11/21 Pages) INTERSIL

11 / 21 page

FN9110.4

April 13, 2007

results in a control loop that behaves more as a voltage

mode controller than as current mode controller.

The minimum amount of capacitance to place at the SLOPE

pin is:

where tON is the On time and VSLOPE is the amount of

voltage to be added as slope compensation to the current

feedback signal. In general, the amount of slope

compensation added is 2 to 3 times the minimum required.

Example:

Assume the inductor current signal presented at the ISENSE

pin decreases 125mV during the Off period, and:

Switching Frequency, Fsw = 250kHz

Duty Cycle, D = 60%

tON = D/Fsw = 0.6/250E3 = 2.4μs

tOFF = (1 - D)/Fsw = 1.6μs

Determine the downslope:

Downslope = 0.125V/1.6

μs = 78mV/μs. Now determine the

amount of voltage that must be added to the current sense

signal by the end of the On time.

Therefore

An appropriate slope compensation capacitance for this

example would be 1/2 to 1/3 the calculated value, or

between 68pF and 33pF.

Overvoltage and Undervoltage Monitor

The OV and UV signals are inputs to a window comparator

used to monitor the input voltage level to the converter. If the

voltage falls outside of the user designated operating range,

a shutdown fault occurs. For OV faults, the supply current,

ICC, is reduced to 200μA for ~295ms at which time recovery

is attempted. If the fault is cleared, a soft-start cycle begins.

Otherwise another shutdown cycle occurs. A UV condition

also results in a shutdown fault, but the device does not

enter the low power mode and no restart delay occurs when

the fault clears.

A resistor divider between VIN and LGND to each input

determines the operational thresholds. The UV threshold

has a fixed hysteresis of 75mV nominal.

Overcurrent Operation

The overcurrent threshold level is set by the voltage applied

at the ISET pin. Setting the overcurrent level may be

accomplished by using a resistor divider network from VREF

to LGND. The ISET threshold should be set at a level that

corresponds to the desired peak output inductor current plus

the additive effects of slope compensation.

Overcurrent delayed shutdown is enabled once the soft-start

cycle is complete. If an overcurrent condition is detected, the

soft-start charging current source is disabled and the

discharging current source is enabled. The soft-start

capacitor is discharged at a rate of 40

μA. At the same time a

μs retriggerable one-shot timer is activated. It remains

active for 50

μs after the overcurrent condition stops. The

soft-start discharge cycle cannot be reset until the one-shot

timer becomes inactive. If the soft-start capacitor discharges

by more then 0.125V to 4.375V, the output is disabled and

the soft-start capacitor is discharged. The output remains

disabled and ICC drops to 200μA for approximately 295ms.

A new soft-start cycle is then initiated. The shutdown and

restart behavior of the OC protection is often referred to as

hic-cup operation due to its repetitive start-up and shutdown

characteristic.

If the overcurrent condition ceases at least 50

μs prior to the

soft-start voltage reaching 4.375V, the soft-start charging

and discharging currents revert to normal operation and the

soft-start voltage is allowed to recover.

Hiccup OC protection may be defeated by setting ISET to a

voltage that exceeds the Error Amplifier current control

voltage, or about 1.5V.

Leading Edge Blanking

The initial 100ns of the current feedback signal input at

ISENSE is removed by the leading edge blanking circuitry.

The blanking period begins when the GATE output leading

edge exceeds 3.0V. Leading edge blanking prevents current

spikes from parasitic elements in the power supply from

causing false trips of the PWM comparator and the

overcurrent comparator.

Fault Conditions

A Fault condition occurs if VREF falls below 4.65V, the OV

input exceeds 2.50V, the UV input falls below 1.45V, or the

junction temperature of the die exceeds ~+130°C. When a

Fault is detected the GATE output is disabled and the soft-

start capacitor is quickly discharged. When the Fault

Time

Downslope

Current Sense Signal

TIME

DOWNSLOPE

CURRENT SENSE SIGNAL

FIGURE 5.

SLOPE

4.24

–

×10

SLOPE

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

•

(EQ. 6)

SLOPE

---

0.078

2.4

•

94mV

(EQ. 7)

SLOPE MIN

()

4.24

–

×10

2.4

–

×10

0.094

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

•

110pF

≈

(EQ. 8)

ISL6721

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