LM2747MTCSLASHNOPB datasheet(21/38 Pages) TI

21 / 38 page

VRAMP

VREF

RFB2

RFB1

CC1

CC2

RC1

RC2

CC3

VIN

LM2747

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SNVS370B – MARCH 2006 – REVISED MARCH 2013

For this example, the maximum drain-to-source voltage applied to either MOSFET is 3.6V. The maximum drive

voltage at the gate of the high-side MOSFET is 3.1V, and the maximum drive voltage for the low-side MOSFET

is 3.3V. Due to the low drive voltages in this example, a MOSFET that turns on fully with 3.1V of gate drive is

needed. For designs of 5A and under, dual MOSFETs in SO-8 provide a good trade-off between size, cost, and

efficiency.

Support Components

CIN2 A small (0.1 to 1 µF) ceramic capacitor should be placed as close as possible to the drain of the high-side

MOSFET and source of the low-side MOSFET (dual MOSFETs make this easy). This capacitor should be

X5R type dielectric or better.

RCC, CCC These are standard filter components designed to ensure smooth DC voltage for the chip supply. RCC

should be 1 to 10

Ω. CCC should 1 µF, X5R type or better.

CBOOT Bootstrap capacitor, typically 100 nF.

RPULL-UP This is a standard pull-up resistor for the open-drain power good signal (PWGD). The recommended

value is 100 k

Ω connected to VCC. If this feature is not necessary, the resistor can be omitted.

A small Schottky diode should be used for the bootstrap. It allows for a minimum drop for both high and

low-side drivers. The MBR0520 or BAT54 work well in most designs.

RCS

Resistor used to set the current limit. Since the design calls for a peak current magnitude (IOUT + (0.5 x

ΔIOUT)) of 4.8A, a safe setting would be 6A. (This is below the saturation current of the output inductor,

which is 7A.) Following the equation from the CURRENT LIMIT section, a 1.3 k

Ω resistor should be used.

RFADJ This resistor is used to set the switching frequency of the chip. The resistor value is approximated from

the Frequency vs Frequency Adjust Resistor curve in the TYPICAL PERFORMANCE

CHARACTERISTICS section. For 300 kHz operation, a 100 k

Ω resistor should be used.

CSS

The soft-start capacitor depends on the user requirements and is calculated based on the equation given

in the section titled START UP/SOFT-START. Therefore, for a 7 ms delay, a 12 nF capacitor is suitable.

Control Loop Compensation

The LM2747 uses voltage-mode (‘VM’) PWM control to correct changes in output voltage due to line and load

transients. VM requires careful small signal compensation of the control loop for achieving high bandwidth and

good phase margin.

The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle

modulator, output inductor, output capacitor, and load. The second part is the error amplifier, which for the

LM2747 is a 9 MHz op-amp used in the classic inverting configuration. Figure 30 shows the regulator and control

loop components.

Figure 30. Power Stage and Error Amp

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