FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
below 85% of the CCM duty cycle. As the load decreases,
Z-MODE OPERATION
this pulse skipping reduces the switching frequency and the
switching losses thus improving efficiency. For example, if a
light load demanded a 30% duty cycle at 2.0MHz, with Z-
mode, this same load will require only
The 34727 operates as a typical fixed frequency, PWM
regulator, at moderate to heavy load currents. As the load is
decreased, such that operation transitions from continuous
conduction mode (CCM) to discontinuous conduction mode
(DCM), the duty cycle is reduced until it approaches 85% of
the full load duty cycle. At this point, the 34727 transitions into
Z-mode operation, where the Z-mode Factor is 0.85. In Z-
mode, the regulator skips pulses whenever the duty cycle is
(0.3/0.85)2 x 2.0MHz = 0.249MHz switching frequency,
hence switching losses will be reduced by almost ten fold.
Figure 15 illustrates the transition to and the exit from Z-
mode.
V
ZERR
V
RAMP
PWM
PWM_Ref.
Z Factor
SW
On Time
Figure 15. Z-mode Operation
internal circuit blocks to lower the device’s quiescent current.
Additionally, the oscillator frequency drops to 250kHz and the
low side switch is turned off, to emulate the operation of an
asynchronous buck converter.
SLEEPY Z-MODE OPERATION
To improve low current efficiency, the 34727 transitions
into the Sleepy Z-mode at load currents of approximately
1.0mA and lower. This is accomplished by powering down
DETAILED FUNCTIONAL DEVICE OPERATION
OVER-CURRENT PROTECTION
SHORT-CIRCUIT PROTECTION
The 34727 implements two layers of protection during
overload conditions. The first is a current limit feature to
prevent the device and external components from damage.
When the peak inductor current reaches the over-current
limit, nominally 900mA, the high side MOSFET turns off to
provide cycle by cycle protection. If the over-current condition
persists and the die temperature surpasses the over-
temperature protection (OTP) threshold, this second layer of
protection shuts down the device.
When a short-circuit condition occurs on the output, typical
regulators will tend to operate at maximum duty cycle. This
condition can saturate the inductor and produce severe peak
currents, resulting in damage to the device. The 34727
avoids this scenario by detecting output voltages below 0.5V.
Upon detection, the part re-starts continuously until the short
circuit condition is removed, or the part surpasses its OTP
threshold.
MC34727
Analog Integrated Circuit Device Data
Freescale Semiconductor
12
FREESCALE [ Freescale ]
