ISL854102DEMO2Z datasheet(14/21 Pages) RENESAS

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ISL854102

FN8870 Rev.1.00

Page 14 of 21

Mar 15, 2019

Light Load Operation

At light loads, converter efficiency can be improved by enabling

variable frequency operation (PFM). Connecting the SYNC pin to

GND allows the controller to choose such operation

automatically when the load current is low. Figure 32 shows the

DCM operation. The IC enters DCM mode when eight consecutive

cycles of inductor current crossing zero are detected. This

corresponds to a load current equal to 1/2 the peak-to-peak

inductor ripple current and set by Equation 2:

where D = duty cycle, fSW = switching frequency, L = inductor

value, IOUT = output loading current, and VOUT = output voltage.

While operating in PFM mode, the regulator controls the output

voltage with a simple comparator and pulsed FET current. A

comparator indicates the point at which FB is equal to the

600mV reference, at which time the regulator begins providing

pulses of current until FB is moved above the 600mV reference

by 1%. The current pulses are approximately 400mA and are

issued at a frequency equal to the converter’s programmed PWM

operating frequency.

Due to the pulsed current nature of PFM mode, the converter can

supply limited current to the load. If load current rises beyond the

limit, VOUT begins to decline. A second comparator signals an FB

voltage 2% lower than the 600mV reference and forces the

converter to return to PWM operation.

Output Voltage Selection

The regulator output voltage is programmed using an external

resistor divider to scale VOUT relative to the internal reference

voltage. The scaled voltage is applied to the inverting input of the

error amplifier; see Figure 34.

The output voltage programming resistor, R3, depends on the

value chosen for the feedback resistor, R2, and the needed

output voltage, VOUT, of the regulator. Equation 3 describes the

relationship between VOUT and resistor values.

If the needed output voltage is 0.6V, then R3 is left unpopulated

and R2 is 0Ω.

Protection Features

The ISL854102 is protected from overcurrent, negative

overcurrent, and over-temperature. The protection circuits

operate automatically.

FIGURE 32. DCM MODE OPERATION WAVEFORMS

CLOCK

VOUT

8 CYCLES

PWM

DCM

PWM

LOAD CURRENT

PULSE SKIP

DCM

FIGURE 33. PWM OPERATION WAVEFORMS

VCOMP

VCSA

DUTY

CYCLE

VOUT

OUT

VOUT 1D

–



2L fSW

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

(EQ. 2

R2x0.6V

VOUT 0.6V

–

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

(EQ. 3)

0.6V

REFERENCE

VOUT

FIGURE 34. EXTERNAL RESISTOR DIVIDER

ISL854102DEMO2Z Datasheet(PDF) 14 Page - Renesas Technology Corp

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