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SC2446AEVB Datasheet(PDF) 17 Page - Semtech Corporation

Part # SC2446AEVB
Description  Dual-Phase, Single or Dual Output Synchronous Step-Down Controller
PDF  27 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2446AEVB Datasheet(HTML) 17 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC2446A
converter output voltage. This non-random error is
caused by the feedback voltage divider ratio. It cannot
be corrected by the feedback loop.
The following table lists a few standard resistor
combinations for realizing some commonly used output
voltages.
)
V
(
o
V
6
.
0
9
.
0
2
.
15
.
1
8
.
15
.
23
.
3
h
/
)
h
-
1
(
2
.
0
8
.
0
4
.
12
6
.
24
6
.
5
)
m
h
O
(
1
o
R
0
0
2
6
0
8
K
4
.
1K
2
K
1
6
.
2K
2
0
.
4K
2
6
.
5
)
m
h
O
(
2
o
R
K
1
K
1
K
1K
1
K
1K
1K
1
Only the voltages in boldface can be precisely set with
standard 1% resistors.
From this table, one may also observe that when the
value
5
.
0
5
.
0
V
h
h
1
o −
=
−
and its multiples fall into the standard resistor value
chart (1%, 5% or so), it is possible to use standard value
resistors to exactly set up the required output voltage
value.
The input bias current of the error amplifier also causes
an error in setting the output voltage. The maximum
inverting input bias currents of error amplifiers 1 and 2
is -250nA. Since the non-inverting input is biased to 0.5V,
the percentage error in the second output voltage will be
–100% · (0.25
µA) · R
o1
R
o2
/[0.5 · (R
o1
+R
o2
) ]. To keep
this error below 0.2%, R
o2
< 4k
Ω.
Loop Compensation
SC2446A uses current-mode control for both step-down
channels. Current-mode control is a dual-loop control
system in which the inductor peak current is loosely
controlled by the inner current-loop. The higher gain outer
loop regulates the output voltage. Since the current loop
makes the inductor appear as a current source, the
complex high-Q poles of the output LC networks is split
into a dominant pole determined by the output capacitor
and the load resistance and a high frequency pole. This
pole-splitting property of current-mode control greatly
simplifies loop compensation.
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 SC2446A the
compensation ramp is made duty-ratio dependent. The
compensation ramp is approximately
.
A
30
*
De
I
D
76
.
1
ramp
µ
=
The slope of the compensation ramp is then
.
A
30
*
f
e
)
D
76
.
1
1
(
S
s
D
76
.
1
e
µ
+
=
The slope of the internal compensation ramp is well 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 synchronous
Buck converter with current mode control is shown in
Figure 12.
k
Figure 12. A simple model of synchronous buck converter
with current mode control.
The transconductance error amplifier (in the SC2446A)
has a gain g
m of 260µA/V. The target of the compensation
design is to select the compensation network consisting
of C
2, C3 and R2, along with the feedback resistors Ro1,
Application Information (Cont.)



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