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Fabricante

EL7571C

Programmable PWM Controller

Elantec -> Intersil 

EL7571C

Programmable PWM Controller

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 exclu-

sively 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 dissipa-

tion 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:

VRIP = IRIP × ESROUT

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 induc-

tance (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:

∆VOUT

=







E

S

RO

U

T

×

∆IOUT

+

ESL

×

dd----it

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 EL7571C incorporates a boot-strap gate drive

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

channel MOSFETs are preferred because of their rela-

tive 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 addi-

tion 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 (below5A),

switching loss is the dominant factor. Switching loss is

determined by:

P = C × V2 × F

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 MOS-

FET driver can not supply the peak current required to

turn the MOSFET on and off fast. This results in addi-

tional 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 maxi-

mum current and handle the power dissipation.

The EL7571C is designed to boot-strap to 12V for 12V

only input converters. In this application, logic level

MOSFETs are not required.

Table below lists a few popular MOSFETs and their crit-

ical specifications.

11

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