EL7571 datasheet(10/19 Pages) RENESAS

10 / 19 page

EL7571

FN7298 Rev 1.00

Page 10 of 19

October 25, 2004

Input Capacitor, C1

In a buck converter, where the output current is greater than

10A, significant demand is placed on the input capacitor.

Under steady state operation, the high side FET conducts only

when it is switched “on” and conducts zero current when it is

turned “off”. The result is a current square wave drawn from the

input supply. Most of this input ripple current is supplied from

the input capacitor C1. The current flow through C1’s

equivalent series resistance (ESR) can heat up the capacitor

and cause premature failure. Maximum input ripple current

occurs when the duty cycle is 50%, a current of IOUT/2 RMS.

Worst case power dissipation is:

where:

ERSIN = input capacitor ESR

For safe and reliable operation, PD must be less than the

capacitor’s data sheet rating.

Input Inductor, L2

The input inductor (L2) isolates switching noise from the input

supply line by diverting buck converter input ripple current into

the input capacitor. Buck regulators generate high levels of

input ripple current because the load is connected directly to

the supply through the top switch every cycle, chopping the

input current between the load current and zero, in proportion

to the duty cycle. The input inductor is critical in high current

applications where the ripple current is similarly high. An

exclusively large input inductor degrades the converter’s load

transient response by limiting the maximum rate of change of

current at the converter input. A 1.5µH input inductor is

sufficient in most applications.

Output Capacitor, C2

During steady state operation, output ripple current is much

less than the input ripple current since current flow is

continuous, either via the top switch or the bottom switch.

Consequently, output capacitor power dissipation is less of a

concern than the input capacitor’s. However, low ESR is still

required for applications with very low output ripple voltage or

transient response requirements. Output ripple voltage is given

by:

where:

IRIP = output ripple current

ESROUT = output capacitor ESR

During a transient response, the output voltage spike is

determined by the ESR and the equivalent series inductance

(ESL) of the output capacitor in addition to the rate of change

and magnitude of the load current step. The output voltage

transient is given by:

where:

ESROUT = output capacitor ESR

ESL = output capacitor ESL

IOUT = output current step

di/dt = rate of change of output current

Power MOSFET, Q1 and Q2

The EL7571 incorporates a boot-strap gate drive scheme to

allow the usage of N-channel MOSFETs. N-channel MOSFETs

are preferred because of their relative low cost and low on

resistance. The largest amount of the power loss occurs in the

power MOSFETs, thus low on resistance should be the

primary characteristic when selecting power MOSFETs. In the

boot-strap gate drive scheme, the gate drive voltage can only

go as high as the supply voltage, therefore in a 5V system, the

MOSFETs must be logic level type, VGS<4.5V. In addition to

on resistance and gate to source threshold, the gate to source

capacitance is also very important. In the region when the

output current is low (below 5A), switching loss is the dominant

factor. Switching loss is determined by:

where:

C is the gate to source capacitance of the MOSFET

V is the supply voltage

F is the switching frequency

Another undesirable reason for a large MOSFET gate to

source capacitance is that the on resistance of the MOSFET

driver can not supply the peak current required to turn the

MOSFET on and off fast. This results in additional MOSFET

conduction loss. As frequency increases, this loss also

increases which leads to more power loss and lower efficiency.

Finally, the MOSFET must be able to conduct the maximum

current and handle the power dissipation.

The EL7571 is designed to boot-strap to 12V for 12V only input

converters. In this application, logic level MOSFETs are not

required.

The following table below lists a few popular MOSFETs and

their critical specifications.

IOUT

-------------





ESRIN



VRIP

IRIP ESROUT



VOUT

ESROUT IOUT ESL

-----









PC V



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