NCP81382
www.onsemi.com
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
NCP81382 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 NCP81382 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 NCP81382 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 NCP81382 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
