SC2618 Semtech, SC2618 Datasheet - Page 6

• Manufacturer Part Number: SC2618
• Description: Miniature PWM Controller for Buck Regulator
• Manufacturer: Semtech

Input Capacitor
Input Capacitor
Bootstrap Circuit
Bootstrap Circuit
Filter for Vcc Supply Power
Filter for Vcc Supply Power
The output capacitor ESR also causes output voltage tran-
sient V
meet output transient criteria, the ESR value should be:
To meet both criteria, the smaller one of above two ESRs
is required.
The output capacitor value also contributes to load tran-
sient response. Based on a worst case where the induc-
tor energy 100% dumps to the output capacitor during
the load transient, the capacitance then can be calcu-
lated by:
Input Capacitor
Input Capacitor
Input Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a synchronous buck converter. This value
is given by:
where I
rent, and D is the duty cycle. Choosing low ESR input
capacitors will help maximize ripple rating for a given size.
Bootstrap Circuit
Bootstrap Circuit
Bootstrap Circuit
The SC2618 uses an external bootstrap circuit to pro-
vide a voltage at BST pin for the top MOSFET drive. This
voltage is held up by a bootstrap capacitor. Typically, it is
recommended to use a 1uF ceramic capacitor with 25V
rating and a commonly available diode IN4148 for the
bootstrap circuit.
Filter for Vcc Supply Power
Filter for Vcc Supply Power
Filter for Vcc Supply Power
For the pin of Vcc, it is recommended to use a 1uF/25V
ceramic capacitor for decoupling. In addition, place a
small resistor (10 ohm) in between Vcc pin and the sup-
ply power for noise reduction.
C
POWER MANAGEMENT
Component Selection (Cont.)
R
I
 2004 Semtech Corp.
RMS
ESR
>
L
<
=
⋅
T
V
V
o
I
I
during a transient load current I
T
T
T
T
1 (
is the load current, I
2
2
−
D
)
⋅
I
IN
2
+
D
⋅
(
I
o
−
IN
I
is the input average cur-
IN
)
2
T
flowing in. To
6
Divider Ratio
Divider Ratio
Divider Ratio
Divider Ratio
Divider Ratio
The top resistor of the voltage divider can be chosen
from 5k to 15k. Then the bottom resistor is found from
where 1.25V is the internal reference voltage of the
SC2618.
R
bot
=
V
O
. 1
−
25
. 1
V
25
V
⋅
R
top
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