LM27402 datasheet(20/32 Pages) NSC

20 / 32 page

30092624

FIGURE 17. VDD Charge Pump Circuit

The circuit in Figure 17 will effectively supply close to the VDD

voltage (4.5V) between the gate and the source of the high-

side MOSFET during the on time. It is recommended to use

a Schottky diode for D

BOOT with sufficient reverse standoff

voltage and continuous current rating. The average current

through this diode is dependent on the gate charge of the

high-side FET and the frequency. It can be calculated using

the following equation

DBOOT is the average current through the DBOOT diode, fSW

(Hz) is the switching frequency and Q

GHS (C) is the gate

charge of the high-side MOSFET. If the input voltage is below

5.5V, it is recommended to connect VDD to the input supply

of the LM27402 through a 1

Ω resistor shown in Figure 18.

This will increase the gate voltage of both the low-side and

high-side FETs.

30092632

FIGURE 18. Tie V

DD to VIN when VIN ≤ 5.5V

POWER / EFFICIENCY CALCULATIONS

The overall efficiency of a buck regulator is simply the ratio of

output power to input power. Accurately predicting the overall

efficiency can be tedious and depends on many variables.

Although power losses can be found in almost every compo-

nent of a buck regulator, the following sections present equa-

tions detailing components with the highest relative power

loss.

MOSFETS

Selecting the correct MOSFET for a design is important to the

overall operation of the circuit. If inappropriate FETs are se-

lected for the application, it can result in poor efficiency, high

temperature issues, shoot-through and other impairments. It

is important to calculate the power dissipation for each MOS-

FET at the maximum output current and make sure the max-

imum allowable power dissipation is not exceeded. MOSFET

datasheets should also specify a junction-to-ambient thermal

resistance (

JA) so the temperature rise can be estimated

from this specification .

Both high-side and low-side FETs contribute significant loss

to the system relative to the other components. The high-side

FET contributes transition switching losses, conduction loss-

es and gate charge losses. The low-side FET also contributes

conduction and gate charge losses, but the FET body diode

voltage drop during deadtime and reverse recovery loss must

also be considered. The transition losses for the low-side FET

are small and usually ignored.

High-Side MOSFET

The next set of equations can be used to calculate the losses

associated with the high-side FET.

CND_HS is the conduction loss of the high-side FET during the

D cycle when current is flowing through the FET on-resis-

tance. A self heating coefficient of 1.3 is included in this

equation to approximate the effects of the R

DS(ON) tempera-

ture coefficient. R

DS(ON)_HS (Ω) is the drain to source resis-

tance, I

OUT (A) is the output current and D is the duty ratio.

SW_HS is the switching power loss during the high-side FET

transition time. V

IN (V) is the input voltage, fSW (Hz) is the

switching frequency, and t

r and tf (s) are the rise and fall times

of the switch-node voltage respectively. P

TOT_HS is the total

power dissipation of the high-side FET.

The gate charge of the high-side MOSFET can greatly affect

the turn-on transition time and therefore efficiency. Further-

more, it is wise to consider the ratio of switching loss to

conduction loss associated with the high-side FET. If the duty

ratio is small and the input voltage is high, it may be beneficial

to tradeoff Q

G for higher RDS(ON) to avoid high switching losses

relative to conduction losses. If the duty ratio is large and the

input voltage is low, then a lower R

DS(ON) FET in tandem with

a higher Q

G may result in less power dissipation.

Low-Side MOSFET

The next set of equations can be used to calculate the losses

due to the low-side FET.

LM27402 Datenblatt(PDF) 20 Page - National Semiconductor (TI)

LM27402 Datenblatt(HTML) 20 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datenblatt Download

Link URL