sp7656en2-l-tr Exar Corporation, sp7656en2-l-tr Datasheet - Page 6

• Manufacturer Part Number: sp7656en2-l-tr
• Description: 600khz High Input Voltage, Internally Compensated Sp7656 Non-synchronous Step-down Dc/dc Voltage Regulator
• Manufacturer: Exar Corporation

Input voltage ripple has three components:
ESR and ESL cause a step voltage drop upon
turn on of the MOSFET. During the on time
the capacitor discharges linearly as it supplies
I
by each term can be calculated from:
Where Trise is the rise time of current through
Capacitor. The total input voltage ripple is sum
of the above:
Output Capacitor Selection
Select the output capacitor for voltage rating,
capacitance and Equivalent Series Resistance
(ESR).
Nominally the voltage rating is selected to be
twice as large as the output voltage. Select the
capacitance to satisfy the specification for
output voltage overshoot/undershoot caused
by current step load. A steady-state output
current I
energy of ½ L I
A sudden decrease in I
surplus in L to be absorbed by C
causes an overshoot in output voltage that is
corrected by the power switch reducing in duty
cycle. Use the following equation to calculate
C
Where:
L is the output inductance
I2 is the step load high current
I1 is the step load low current
V
V
2/19/08 SP7656 PowerBlox
OUT
os
OUT
OUT
-I
is output voltage including overshoot
:
in

is steady state output voltage


. The contribution to Input voltage ripple
V
V
V
,
OUT
,
,
Cin
Cout
ESL
ESR
corresponds to inductor stored

OUT



Iout
ESL
ESR
L
2
.




Vos
Vout
fs


Iout
I
OUT
Iout
2
Cin
2
2


(
forces the energy
Vin
Vout

Trise

I
Vin
1
0
0
2
5 .
5 .

2
2
Vout




Irip
Irip
OUT

)

. This
6
Output voltage undershoot calculation is more
complicated. Test results for SP7656 buck
circuits show that undershoot is approximately
equal to overshoot. Therefore above equation
provides a satisfactory method for calculating
C
ripple (V
components to the output ripple voltage: The
first component arises from charge transferred
to and from C
second component is due to inductor ripple
current flowing through the output capacitor’s
ESR. It can be calculated from:
Where:
I
fs is switching frequency
C
Note that a smaller inductor results in a higher
inductor ripple current, therefore requiring a
larger COUT and/or lower ESR in order to
meet the output voltage ripple requirement.
Schottky Rectifier Selection
Select the Schottky based on the voltage
rating VR, Forward voltage V
resistance
application the Schottky is conducting most of
the time and its conduction losses are the
largest component of losses in the converter.
Conduction losses can be estimated from:
where:
V
The AC losses from the switching capacitance
of a Schottky are negligible and can be
ignored.
RIP
f
OUT
OUT
is diode forward voltage at I
Vrip
is inductor ripple current
. Select ESR such that output voltage
is output capacitor calculated above
RIP

Pc
Irip
) specification is met. There are two
R

thja
Vf
OUT
.
Copyright 2007 EXAR Corporation
ESR
Iout
For
during each cycle. The
2




a
1

 

8

low
Vout
Cout
OUT
Vin
f
, and thermal
1
duty cycle



fs

 

2