sc4508a Semtech Corporation, sc4508a Datasheet - Page 14

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

The inner current-loop is unstable (sub-harmonic
oscillation) unless the inductor current up-slope is steeper
than the inductor current down-slope. For stable
operation above 50% duty-cycle, a compensation ramp
is added to the sensed-current. In the SC4508A the
compensation ramp is made switching frequency
dependent. The slope of the compensation ramp is:
The slope of the internal compensation ramp is above
the minimal slope requirement for current loop stability
and is sufficient for all the applications. With the inner
current loop stable, the output voltage is then regulated
with the outer voltage feedback loop. A simplified
equivalent circuit model of the buck converter with
current mode control is shown in Figure 7.
Figure 7. A simple model of buck converter with current
mode control.
The transconductance error amplifier (in the SC4508A)
has a gain (g
compensation design is to select the compensation
network consisting of C
feedback resistors R
such that the converter output voltage is regulated with
satisfactory dynamic performance.
With the output voltage V
and the feedback resistor values are determined using
the equations given in the “Output Voltage Setting”
section with:
Application Information (Cont.)
POWER MANAGEMENT
2007 Semtech Corp.
k
m
) of 100µA/V. The target of the
S
o1
e
, R
o2
500
2
o
, C
and the current sensing gain,
known, the feedback gain h
f *
3
and R
s
mV
2
, along with the
14
For the rated output current I
k is fixed as:
From Figure 7, the transfer function from the voltage
error amplifier output (v
where, the single dominant pole is:
and the zero due to the output capacitor ESR is:
The dominant pole moves as output load varies.
The controller transfer function (from the converter
output (v
where
and
The loop transfer function is then:
To simplify design, we assume that C
selects S
frequency (f
determines the converter dynamic bandwidth. With these
o
) to the voltage error amplifier output (v
p1
c
=S
). It is noted that the crossover frequency
V
V
z2
G
o
c
(
(
c
and specifies the loop crossover
) s
) s
(
s
) s
p
s
:
1
p
s
2
G
s
T(s)=G
z
k
( s
c
1
R (
z
h
vc
2
) to the converter output (v
C
R
(
o
) s
2
R
2
8
g
R
0
m
C
V
oesr
1
R
5 .
R
C
vc
o
2
1
C
o
1
1
2
1
, the current sensing gain
C
oesr
(s)G
kR
S
2
C
3
C
2
)
o
C
3
o
C )
1
1
3
C
1
1
(s)h
o
s
s
s
s
z
p
s
s
SC4508A
2
2
s
s
1 z
p
3
1
<<C
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2
, R
oesr
c
<<R
) is:
o
)is:
o,