NCV8925

http://onsemi.com

9

DC/DC OPERATION DESCRIPTION

Detailed Description

The NCV8925 uses a constant frequency, voltage mode

step-down architecture. Both the main (P-Channel

MOSFET) and synchronous (N-Channel MOSFET)

switches are internal.

The output voltage is set by an external resistor divider

in the range of 0.9 V to 3.3 V and can source at least

600 mA.

The NCV8925 works with two modes of operation;

PWM/PFM depending on the current required. In PWM

mode, the device can supply voltage with a tolerance of

"3% and 90% efficiency or better. Lighter load currents

cause the device to automatically switch into PFM mode to

reduce current consumption and extended battery life.

Additional features include soft-start, undervoltage

protection, current overload protection, and thermal

shutdown protection. As shown in Figure 2, only six

external components are required. The part uses an internal

reference voltage of 0.6 V. It is recommended to keep

NCV8925 in shutdown mode until the input voltage is

2.7 V or higher.

PWM Operating Mode

In this mode, the output voltage of the device is regulated

by modulating the on-time pulse width of the main switch

Q1 at a fixed 3 MHz frequency.

The switching of the PMOS Q1 is controlled by a

flip-flop driven by the internal oscillator and a comparator

that compares the error signal from an error amplifier with

the sum of the sensed current signal and compensation

ramp.

At the beginning of each cycle, the main switch Q1 is

turned ON by the rising edge of the internal oscillator

clock. The inductor current ramps up until the sum of the

current sense signal and compensation ramp becomes

higher than the error amplifier's voltage. Once this has

occurred, the PWM comparator resets the flip-flop, Q1 is

turned OFF while the synchronous switch Q2 is turned ON.

Q2 replaces the external Schottky diode to reduce the

conduction loss and improve the efficiency. To avoid

overall power loss, a certain amount of dead time is

introduced to ensure Q1 is completely turned OFF before

Q2 is being turned ON.

Figure 26. PWM Switching Waveform

200 ns/div

PFM Operating Mode

Under light load conditions, the NCV8925 enters in low

current

PFM

mode

operation

to

reduce

power

consumption. The output regulation is implemented by

pulse frequency modulation. If the output voltage drops

below the threshold of PFM comparator, a new cycle will

be initiated by the PFM comparator to turn on the switch

Q1. Q1 remains ON during the minimum on time of the

structure while Q2 is in its current source mode. The peak

inductor current depends upon the drop between input and

output voltage. After a short dead time delay where Q1 is

switched OFF, Q2 is turned in its ON state. The negative

current detector will detect when the inductor current drops

below zero and sends the signal to turn Q2 to current source

mode to prevent a too large deregulation of the output

voltage. When the output voltage falls below the threshold

of the PFM comparator, a new cycle starts immediately.

Figure 27. PFM Switching Waveforms