ISL68227IRAZ datasheet(20/100 Pages) RENESAS

20 / 100 page

ISL68227

4. Operating the Device

FN9333 Rev.1.00

Page 20 of 99

Oct.1.20

Any command to change the output voltage set point uses all phases, including VOUT transition events. After the

output voltage change is complete, phases begin dropping as configured.

To ensure dropped phases have sufficient bootstrap capacitor charge to turn on the high-side MOSFET after a

long period of inactivity, a bootstrap refresh function periodically turns on the low-side MOSFET of each dropped

phase to refresh the bootstrap capacitor.

To produce the most optimal efficiency across a wide range of output loading, the modulator supports automatic

dropping or adding of phases and diode emulation (in a single-phase state). Use of APD and diode emulation are

optional. If automatic phase dropping is enabled, the number of active phases at anytime is determined only by

load current.

4.7

Diode Emulation and PFM Operation

As described in Phase Configuration and Automatic Phase Dropping, the ISL68227 supports APD to optimize

phase usage as load demand decreases. When the regulator drops to 1-phase operation, it supports diode

emulation and Pulse Frequency Modulation (PFM) operation to further maximize efficiency performance.

Traditionally, use of such efficiency boosting techniques has come at the expense of transient response, but the

ISL68227 is able to meet all transient demands directly from diode emulation/PFM operation.

Diode emulation and PFM operation are supported when a single phase is active. If constant frequency operation

is needed at light loads, the feature can be disabled. If enabled, the low-side MOSFET conducts when the current

is flowing from source-to-drain and does not allow a reverse current, emulating an ideal diode. As Figure 8 shows,

when the inductor current is positive, the LGATE is held on and allows current to flow in the low resistance

channel of the LFET. When current reaches zero, the LFET is turned off to prevent a reverse current in the

inductor. The controller modulates the LFET state through the PWM pin of the respective regulator channel by

tri-stating the PWM when the load current reaches zero, which commands the MOSFET driver to turn off both the

HFET and LFET.

In single-phase diode emulation operation, the IC delivers inductor current pulses with a user programmed pulse

width. By programming the pulse width, the output voltage ripple can be tuned to meet expectations for any

system type. Pulse frequency is then modulated to maintain output voltage regulation, which is shown in Figure 9.

The transition from single phase PFM to multiphase constant frequency operation is managed seamlessly by the

IC.

4.8

Diode Braking

Applications that support loads with large transient current demands often have significant output voltage

overshoot when the load current demand drops suddenly. In some cases, diode braking may allow overshoot

reduction at the expense of thermal dissipation in the low-side MOSFET.

4.9

Switching Frequency

Switching frequency is independently programmable on each rail from 200kHz to 2MHz.

Figure 8. Diode Emulation PWM Signaling

Figure 9. Single Phase Light Load PFM Operation

ISL68227IRAZ Datasheet(PDF) 20 Page - Renesas Technology Corp

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