ISL85415

5. Detailed Description

FN8373 Rev.5.01

Page 20 of 30

Jun.4.20

Output voltage is regulated as the error amplifier varies VCOMP and therefore the output inductor current. The

error amplifier is a trans-conductance type and its output (COMP) is terminated with a series RC network to GND.

This termination is internal (150k/54pF) if the COMP pin is tied to VCC. Additionally, the trans-conductance for

COMP = VCC is 50µs vs 220µs for external RC connection. Its non-inverting input is internally connected to a

600mV reference voltage and its inverting input is connected to the output voltage via the FB pin and its

associated divider network.

5.5

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.

Figures 58 shows the DCM operation. The ISL85415 enters the DCM mode of operation when eight consecutive

cycles of inductor current crossing zero are detected. This event 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, VOUT = and

output voltage.

While operating in PFM mode, the regulator controls the output voltage with a simple comparator and pulsed FET

current. A comparator signals 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

Figure 57. PWM Operation Waveforms

Figure 58. DCM Mode Operation Waveforms

VCOMP

VCSA

Duty

Cycle

IL

VOUT

IOUT

VOUT 1D

–



2LfSW

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

=

(EQ. 2)

Clock

IL

VOUT

0

8 Cycles

PWM

DCM

PWM

Load Current

Pulse Skip

DCM