XRP7714 Exar, XRP7714 Datasheet - Page 25

• Manufacturer Part Number: XRP7714
• Description: Quad Channel Digital PWM Step Down Controller
• Manufacturer: Exar

When trying to optimize control loop bandwidth, particularly at switching frequencies below
600kHz, an effective ESR in the range of 7 to 20mohm can help significantly. The Digital Power
Studio design tool is used to verify what will work best in your application.
Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is
nominally selected to be at least twice the input voltage. The RMS value of input capacitor current,
assuming a low inductor ripple current, can be approximated as:
Where:
Iin is the RMS input current
Iout is the DC output current
D is the duty cycle
In general, the total input voltage ripple should be kept below 1.5% of VIN. The input voltage
ripple also has two major components: the voltage drop on the main capacitor
voltage drop due to ESR -
calculated from:
Total input voltage ripple is the sum of the above:
Selecting MOSFETs with lower Rdson reduces conduction losses at the expense of increased
switching losses. A simplified expression for conduction losses is given by:
MOSFET’s junction temperature can be estimated from:
This assumes that the switching loss is the same as the conduction loss.
thermal resistance from junction to ambient.
© 2010 Exar Corporation
Power MOSFETs Selection
•
Input Capacitor Selection
Δ
V
ESR
. The contribution to Input voltage ripple by each term can be
Q
Q
Δ
u
u
Δ
V
a
a
V
P
T
ESR
Δ
d
d
cond
Cin
j
V
= 2
C
C
Tot
=
=
=
h
h
ESR
P
=
I
I
cond
a
a
out
out
Δ
25/27
n
n
V
V
2
R
⋅
n
n
out
Cin
(
f
thja
⋅
e
e
I
s
R
C
out
(
l
l
V
+
dson
+
in
D
in
D
+
Δ
V
T
1
V
−
i
in
⋅
ambient
i
0
2
g
g
V
ESR
V
5 .
V
out
i
i
in
out
I
t
t
rip
a
a
)
l
l
)
P
P
W
W
M
M
S
S
t
t
e
e
R
p
p
thja
D
D
is the total MOSFET
o
o
w
w
n
n
www.DataSheet4U.com
Δ
X
X
C
C
V
R
R
o
o
Cin
P
n
P
n
Rev. 1.1.3
t
t
and the
7
7
r
r
o
o
7
7
l
l
1
1
l
l
e
e
4
4
r
r