SC2602L Semtech Corporation, SC2602L Datasheet - Page 11

• Manufacturer Part Number: SC2602L
• Description: Synchronous Voltage Mode Controller
• Manufacturer: Semtech Corporation

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Fig. 2. SC2602L small signal model.
The control model of SC2602L control loop small signal
can be depicted in Fig. 2. This model can also be used in
SPICE kind of simulator to generate loop gain Bode plots.
The bandgap reference is 0.8V and trimmed to +/-1%
accuracy. The desired output voltage can be achieved
by setting the resistive divider network, R1 and R2.
The error amplifier is transconductance type with fixed
gain of:
The compensation network includes a resistor and a
capacitor in series, which terminates from the output of
the error amplifier to the ground.
This device uses voltage mode control with input volt-
age feed forward. The peak-to-peak ramp voltage is
proportional to the input voltage, which results in an ex-
cellent performance to reject input voltage variation. The
PWM gain is inversion of the ramp amplitude, and this
gain is given by:
where the ramp amplitude (peak-to-peak) is 1.0 volts
when input voltage is 12 volts.
The total control loop-gain can then be derived as
follows:
T s ( ) T o
 2005 Semtech Corp.
POWER MANAGEMENT
1.25Vdc
0.8Vdc
0.5Vdc
Vbg
.
1 s R . C .
s R . C .
EA
T
o
:=
G m
G pwm
.
1 s R c C o
G
R
C
m
Gpwm
⋅
.
G
1.8 mA
pwm
V
.
Vin
.
V ramp
⋅
V
1
1 s R c
in
R o
R ⋅
L
⋅
.



L
V
V
.
s
C o
bg
2
o
L . C o


.
Rc
Co
.
1
Ro
R c
R o
R1
R2
11
The task here is to properly choose the compensation
network for a nicely shaped loop-gain Bode plot. The
following design procedures are recommended to ac-
complish the goal:
(1) Calculate the corner frequency of the output filter:
(2) Calculate the ESR zero frequency of the output filter
capacitor:
(3) Check that the ESR zero frequency is not too high.
If this condition is not met, the compensation structure
may not provide loop stability. The solution is to add
some electrolytic capacitors to the output capacitor bank
to correct the output filter corner frequency and the ESR
zero frequency. In some cases, the filter inductance
may also need to be adjusted to shift the filter corner
frequency. It is not recommended to use only high fre-
quency multi-layer ceramic capacitors for output filter.
(4) Choose the loop gain cross over frequency (0 dB
frequency). It is recommended that the crossover fre-
quency is always less than one fifth of the switching
frequency :
If the transient specification is not stringent, it is better to
choose a crossover frequency that is less than one tenth
of the switching frequency for good noise immunity. The
resistor in the compensation network can then be cal-
culated as:
when
R
:=
G
pwm
⋅
F
1
V
F
F
F
F
o
in
esr
o
esr
x_over
<
⋅
G
:=
:=
F
m
<
esr
2 π ⋅ L C
2 π ⋅ R
⋅
F



≤
<
⋅
5
sw
F
⋅
F
F
1
F
esr
1
o
5
sw
c
x_over
⋅
⋅
C


o
2
o
⋅



F
F
x_over
SC2602L
esr
www.semtech.com


⋅



V
V
bg
o

