sc4508a Semtech Corporation, sc4508a Datasheet - Page 10

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

Req and Ceq are both functions of frequency. For rigorous
design, the equivalent ESR should be evaluated at the
ripple frequency for voltage ripple calculation when both
ceramic and electrolytic capacitors are used. If R
= R
independent and
Input Capacitor (C
The input supply to the converter usually comes from a
pre-regulator. Since the input supply is not ideal, input
capacitors are needed to filter the current pulses at the
switching frequency. A simple buck converter is shown in
Figure 3.
In Figure 3 the DC input voltage source has an internal
impedance R
R
voltage ripple and input voltage ripple are shown in Figure
4.
Figure 4. Typical waveforms at converter input.
Application Information (Cont.)
POWER MANAGEMENT
esr
2007 Semtech Corp.
. MOSFET and input capacitor current waveforms, ESR
1
Figure 3. A simple model for the converter input
and C
1a
= C
in
and the input capacitor C
R
1b
eq
= C
= 1/2 R
in
)
1
, then R
1
and C
eq
and C
eq
= 2C
eq
will be frequency-
in
1
has an ESR of
1a
= R
1b
10
It can be seen that the current in the input capacitor
pulses with high di/dt. Capacitors with low ESL should be
used. It is also important to place the input capacitor
close to the MOSFETs on the PC board to reduce trace
inductances around the pulse current loop.
The RMS value of the capacitor current is approximately
The power dissipated in the input capacitors is then:
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10
temperature rise. Many manufacturers specify the
maximum allowable ripple current (RMS) rating of the
capacitor at a given ripple frequency and ambient
temperature. The input capacitance should be high
enough to handle the ripple current. For higher power
applications, multiple capacitors are placed in parallel to
increase the ripple current handling capability.
Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is:
The peak-to-peak input voltage ripple due to the capacitor
is:
From these two expressions, C
the input voltage ripple specification.
Power MOSFETs Selection
Main considerations in selecting the MOSFETs are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFETs are directly related to
the total gate charge (C
(R
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 5.
ds(on)
). In order to judge the performance of MOSFETs,
I
Cin
I
o
v
D
ESR
[(
1
P
Cin
R
12
v
= I
esr
2
C
g
)(
) and channel on-resistance
1 (
Cin
1
C
DI
2
R
in
2
D
dc
IN
esr
f
I )
s
)
dc
2
can be found to meet
D
2
SC4508A
1 (
www.semtech.com
D
] )
o
C of