SMB119 datasheet(22/34 Pages) SUMMIT

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SMB119

Summit Microelectronics, Inc

BUCK CONVERTERS

The SMB119 has three synchronous buck converters

with integrated p-channel MOSFETS and a driver for an

external NFET, see Figure 14. Each channel has an

output voltage range from the input supply to

approximately 0.5V.

Buck Channel Asynchronous Operation

The Buck converters use either a constant frequency or

variable frequency current mode control technique.

During the fixed frequency PWM mode of operation, the

converter switches at a fixed frequency and modulates

the duty cycle to attain the correct output voltage. This

can lead to “charge shuttling” under light load

conditions were the charge transferred to the output

capacitor during the on time of the PFET is discharged

to ground during the on time of the NFET. This mode of

operation is desirable in situations requiring low voltage

ripple, the ability to sink current, or a known switching

frequency for all loads.

During the PFM mode of operation the converter

operates asynchronously where the NFET is held off

and the body diode of the FET is used as a “catch”

diode; preventing the voltage on the switch node from

falling below ground by more than a diode drop. It is

desirable to operate asynchronously under light load so

that charge shuttling does not occur. The asynchronous

operation allows the converter to only switch when the

voltage falls below the error amplifier reference voltage.

While it is advantageous to operate asynchronously for

light load currents, it is less efficient for moderate loads

where the power loss across the forward voltage drop

of the diode leads to decreased efficiency. To increase

the efficiency for these moderate load conditions an

external schottky diode can be placed in parallel with

the body diode of the FET.

To maximize the converter efficiency for both light and

heavy loads the Buck converters automatically switch

from PFM to PWM mode. The PWM to PFM crossover

is accomplished by observing the voltage on the COMP

pin, the voltage on the COMP pin is directly

proportional to the load current. When the voltage on

the COMP pin falls below a programmable reference,

the converter operates in PFM. The NFET driver will

stay in the off state until the voltage on the COMP pin

rises above the PFM to PWM crossover voltage.

Each

channel

has

over

current

protection

mechanism. When a channel reaches its current limit,

the output voltage will be reduced as the load rises.

This is accomplished by clamping the COMP node to

one of four programmable settings. The over-current

level can be programmed to four different levels by

clamping the error amplifier's output voltage to a

programmable voltage.

All current limits and PFM to PWM crossover currents

are calculated by the GUI interface.

The output of all Buck converters is determined by the

portion of the switching period for which the inductor

voltage is at the converter supply voltage; this

percentage is referred to as the duty cycle. For a Buck

channel operating synchronously, duty cycle and the

output voltage are related by equation 1 below:

Equation 1:

Vin

Each Buck converter can operate up to 100% duty

cycle allowing the output to equal the input.

The

minimum voltage is determined by the minimum duty

cycle listed in the electrical specifications section. For a

Buck converter operating in PFM mode the duty cycle

is essentially 0% implying that the output can go to

ground.

Each converter has a separate VIN input used to power

the converter. This supply attaches to the source of the

integrated PFET. It is important to connect an input (or

Bulk) as close to the VIN pin as possible. For

information on the type of capacitor to use, refer to the

component selection section.

APPLICATIONS INFORMATION (CONTINUED)

VIN

DRVL

PWM

COMP

Figure 14 – Buck channel with internal PFET.

SMB119 Datasheet(PDF) 22 Page - Summit Microelectronics, Inc.

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