el7571c ETC-unknow, el7571c Datasheet - Page 11

• Manufacturer Part Number: el7571c
• Description: Programmable Controller
• Manufacturer: ETC-unknow

For safe and reliable operation, P
capacitor’s data sheet rating.
Input Inductor, L
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, C
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:
where:
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:
where:
I
ESR
ESR
RIP
OUT
OUT
= output ripple current
= output capacitor ESR
= output capacitor ESR
V
OUT
=
V
RIP
2
ESR
=
2
OUT
I
RIP
ESR
I
OUT
D
OUT
must be less than the
+
ESL
d
----
d
i
t
11
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:
where:
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.
ESL = output capacitor ESL
d
C is the gate to source capacitance of the MOSFET
V is the supply voltage
F is the switching frequency
i
I
/d
OUT
t
= rate of change of output current
= output current step
Programmable PWM Controller
P
=
C
V
2
F
EL7571C