SC2621A Semtech Corporation, SC2621A Datasheet - Page 8

• Manufacturer Part Number: SC2621A
• Description: Voltage Mode PWM Controller
• Manufacturer: Semtech Corporation

Input Capacitor
Input Capacitor
MOSFET for Linear Regulator
MOSFET for Linear Regulator
Bootstrap Circuit
Bootstrap Circuit
Filters for Supply Power
Filters for Supply Power
CONTROL LOOP DESIGN
CONTROL LOOP DESIGN
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.
MOSFET for Linear Regulator
MOSFET for Linear Regulator
MOSFET for Linear Regulator
The MOSFET in linear regulator operates in linear region
with really high power loss. A device with a suitable pack-
age has to be selected to handle the loss. To prevent too
high load current during short circuit, the R
MOSFET should not be selected too low. A good choice is
to select a MOSFET so that it is almost fully turned on at
maximum load current. For example, in a LDO design with
3.3V in and 1.5V/2A out, a MOSFET with 600 to 800m-
ohm R
Bootstrap Circuit
Bootstrap Circuit
Bootstrap Circuit
The SC2621A uses an external bootstrap circuit to pro-
vide a voltage at BST pin for the top MOSFET drive. This
voltage, referring to the Phase Node, is held up by a
bootstrap capacitor. Typically, it is recommended to use
a 1uF ceramic capacitor with 16V rating and a commonly
available diode IN4148 for the bootstrap circuit.
Filters for Supply Power
Filters for Supply Power
Filters for Supply Power
For each pin of DRV and Vcc, it is recommended to use a
1uF/16V ceramic capacitor for decoupling. In addition,
place a small resistor (10 ohm) in between Vcc pin and
the supply power for noise reduction.
CONTROL LOOP DESIGN
CONTROL LOOP DESIGN
CONTROL LOOP DESIGN
The goal of compensation is to shape the frequency re-
sponse charateristics of the buck converter to achieve a
better DC accuracy and a faster transient response for
the output voltage, while maintaining the loop stability.
The block diagram in Fig. 3 represents the control loop
of a buck converter designed with the SC2621A. The
control loop consists of a compensator, a PWM modula-
tor, and a LC filter.
The LC filter and PWM modulator represent the small
signal model of the buck converter operating at fixed
POWER MANAGEMENT
Applications Information (Cont.)
I
RMS
2007 Semtech Corp.
www.DataSheet4U.com
=
dson
o
1 (
is the load current, I
-
can be chosen.
D
)
×
I
IN
2
+
D
×
(
I
o
-
IN
I
is the input average cur-
IN
)
2
dson
of the
8
switching frequency. The transfer function of the model
is given by:
where V
of the 500kHz ramp, and R is the equivalent load.
Fig. 3. Block diagram of the control loop
The model is a second order system with a finite DC gain,
a complex pole pair at Fo, and an ESR zero at Fz, as
shown in Fig. 4. The locations of the poles and zero are
determined by:
The compensator in Fig. 3 includes an error amplifier and
impedance networks Zf and Zs. It is implemented by the
circuit in Fig. 5. The compensator provides an integrator,
double poles and double zeros. As shown in Fig. 4, the
integrator is used to boost the gain at low frequency.
Two zeros are introduced to compensate excessive phase
lag at the loop gain crossover due to the integrator
(-90deg) and complex pole pair (-180deg). Two high fre-
quency poles are designed to compensate the ESR zero
and attenuate high frequency noise.
V
V
F
F
O
C
O
Z
Zs
=
=
=
V
R
V
FB
IN
ESR
m
LC
1
IN
1
REF
×
C
Zf
is the power rail voltage, Vm is the amplitude
1
+
-
+
EA
1
SC2621A AND MOSFET S
sL
+
Vc
/
sR
R
COMP
ESR
+
s
C
MODULAT OR
2
LC
PWM
OUT
L
SC2621A
Co
Resr
www.semtech.com
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