NCP81381

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11

For Use with Controllers with 3−State PWM and No Zero Current Detection Capability:

Table 3. LOGIC TABLE − 3−STATE PWM CONTROLLERS WITH NO ZCD

PWM

SMOD#

ZCD_EN

GH

GL

H

H

H

ON

OFF

M

H

H

OFF

ZCD

L

H

H

OFF

ON

This section describes operation with controllers that are

capable of 3 states in their PWM output and relies on the

NCP81381 to conduct zero current detection during

discontinuous conduction mode (DCM).

The SMOD# pin needs to either be set to 5 V or left

disconnected. The NCP81381 has an internal pull−up

resistor that connects to VCC that sets SMOD# to the logic

high state if this pin is disconnected.

The ZCD_EN pin needs to either be set to 5 V or left

disconnected. The NCP81381 has an internal pull−up

resistor connected to VCC that will set ZCD_EN to the logic

high state if this pin is left disconnected.

To operate the buck converter in continuous conduction

mode (CCM), PWM needs to switch between the logic high

and low states. To enter into DCM, PWM needs to be

switched to the mid−state.

Whenever PWM transitions to mid−state, GH turns off

and GL turns on. GL stays on for the duration of the

de−bounce timer and ZCD blanking timers. Once these

timers expire, the NCP81381 monitors the SW voltage and

turns GL off when SW exceeds the ZCD threshold voltage.

By turning off the LS FET, the body diode of the LS FET

allows any positive current to go to zero but prevents

negative current from conducting.

Figure 9. Timing Diagram − 3−state PWM Controller, No ZCD