34726 datasheet(12/19 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34726

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Z-MODE OPERATION

The 34726 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 34726 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

below 85% of the CCM duty cycle. As the load decreases,

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

(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.

Figure 15. Z-mode Operation

SLEEPY Z-MODE OPERATION

To improve low current efficiency, the 34726 transitions

into the Sleepy Z-mode at load currents of approximately

1.0mA and lower. This is accomplished by powering down

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.

DETAILED FUNCTIONAL DEVICE OPERATION

OVER-CURRENT PROTECTION

The 34726 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 450mA, 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.

SHORT-CIRCUIT PROTECTION

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 34726

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.

PWM

Z Factor

On Time

PWM_Ref.

VRAMP

VZERR

34726 Datasheet(PDF) 12 Page - Freescale Semiconductor, Inc

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