sc2542 Semtech Corporation, sc2542 Datasheet - Page 15

• Manufacturer Part Number: sc2542
• Description: Sc2542 High Performance Wide Input Rangedual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

Input voltage range: Vin,min and Vin,max
Input voltage ripple (peak-to-peak): DVin
Output voltage: Vo
Output voltage accuracy: e
Output voltage ripple (peak-to-peak): DVo
Nominal output (load) current: Io
Maximum output current limit: Io,max
Output (load) current transient slew rate: dIo/dt (A/s)
Circuit efficiency:
Inductor (L) and Ripple Current
Both step-down controllers in the SC2542 operate in
synchronous continuous-conduction mode (CCM)
regardless except in light-load mode. The output
inductor selection/design is based on the output DC
and transient requirements. Both output current and
voltage ripples are reduced with larger inductance but
it takes longer to change the inductor current during
load transients. Conversely smaller inductance results
in lower DC copper losses but the AC core losses (flux
swing) and the winding AC resistance losses are higher.
A compromise is to choose the inductance such that
peak-to-peak inductor ripple-current is 20% to 30% of
the rated output load current.
Assuming that the inductor current ripple (peak-to-
peak) value is
The peak current in the inductor becomes:
and RMS current is:
The followings are to be considered when choosing in-
ductors.
a) Inductor core material: For higher efficiency applica-
tions above 300 KHz, ferrite, Kool-Mu and polypermalloy
materials should be used. Low-cost powdered iron cores
can be used for cost sensitive-applications below 300
KHz but with attendant higher core losses.
b) Select inductance value: Sometimes the calculated
inductance value is not available off-the-shelf. The de-
signer can choose the adjacent (larger) standard induc-
POWER MANAGEMENT
Applications Information (Cont.)
2005 Semtech Corp.
Io, the inductance value will then be:
I
, L
L
rms
1 (
V
I
O
O
/
(1
) 2
I F
O S
1
I *
D
O
12
2
)
15
tance value. The inductance varies with temperature and
DC current. It is a good engineering practice to re-evalu-
ate the resultant current ripple at the rated DC output
current.
c) Current rating: The saturation current of the inductor
should be at least 1.5 times of the peak inductor cur-
rent under all conditions.
Output Capacitor (Co) and Vout Ripple
The output capacitor provides output current filtering in
steady state and serves as a reservoir during load tran-
sient. The output capacitor can be modeled as an ideal
capacitor in series with its parasitic ESR and ESL as shown
in Figure 13.
Figure 13. An equivalent circuit of output.
If the current through the branch is ib(t), the voltages
across the terminals will then be:
This basic equation illustrates the effects of ESR, ESL,
and Co on the output voltage.
The first term is the DC voltage across Co at time t = 0.
The second term is the voltage variation caused by the
charge balance between the load and the converter out-
put. The third term is voltage ripple due to ESL and the
fourth term is the voltage ripple due to ESR. The total
output voltage ripple is then a vector sum of the last
three terms. Since the inductor current is a triangular
waveform with peak-to-peak value ä*Io, the ripple-volt-
age caused by inductor current ripples is:
the ripple-voltage due to ESL is:
and the ESR ripple-voltage is:
V t
O
( )
V
O
Co
1
0
t
V
i t dt Lesl
V
b
ESL
( )
V
ESR
C
L
8
R
esl
C
esr
f
I
O
s
O
f
s
D
I
I
O
,
O
di t
,
dt
b
( )
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SC2542
Re
sri t
b
( ).