sc4508a Semtech Corporation, sc4508a Datasheet - Page 13

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

When C
during this period, the converter does not start switching
until SS/EN reaches 1.4V.
c) The effective start-up time is:
Assuming inductor current hitting current limit for 32
cycles after SS/EN reaches 1.4V and fs=200KHz. The
average inductor current is then:
I
capacitor value. The converter will not overheat in hiccup.
Setting the Output Voltage
The non-inverting input of the error amplifier is brought
out as a device pin (Pin 9) to which the user can connect
Pin 4 or an external voltage reference. A simple voltage
divider (R
output voltage. In buck converter, the voltage feedback
gain (h=0.5/V
as:
Once either R
calculated for the desired output voltage V
number of standard resistance values is limited, the
calculated resistance may not be available as a standard
value resistor. As a result, there will be a set error in the
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.
Application Information (Cont.)
Leff
POWER MANAGEMENT
2007 Semtech Corp.
SS
0.025 I
o1
= 0.1
at top and R
o
µ
o1
LIM
) is related to the divider resistors value
F, t
t
or R
ssr
and is independent of the soft start
ssr2
I
Leff
2
t
R
sse
is calculated as 2.5ms. Note that
o2
o2
C
I
o2
LIM
SS
is chosen, the other can be
at bottom) sets the converter
200
1
1 (
t
ssr
h
4 .
32
h
1
20
KHz
t
sse
R
0
t
o1
ssr
A
9 .
2
V )
o
. Since the
13
Only the voltages in boldface can be precisely set with
standard 1% resistors.
From this table, one may also observe that when the
value
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.
In buck-boost converter, output voltage is set by
The input bias current of the error amplifier also causes
an error in setting the output voltage. The inverting input
bias currents of error amplifiers is -100nA. Since the
non-inverting input is biased to 0.5V in buck converter,
the percentage error in the second output voltage will be
–100% · (0.1
this error below 0.2%, R
Loop Compensation
SC4508A is a current-mode controller. 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.
(
V
R
R
- 1
o
1 o
2 o
) h
(
) V
h /
O (
O (
h
h
) m
) m
µ
1
0
0
2
A) · R
6 .
2 .
0 0
K
1
0
0
8
9 .
8 .
6 0
K
o1
1
R
h
o2
R
h
1
1
1
1
o1
o2
2 .
4 .
4 .
K
K
//R
V
0.5
V
/[0.5 · (R
o
O
1
1
2
2
0
5 .
K
o2
K
5 .
R
0
o1
5 .
1
1
< 10k .
2
. 2
8 .
K
6 .
1 6
o1
K
SC4508A
+R
1
www.semtech.com
2
4
. 4
K
5 .
o2
2 0
K
) ]. To keep
3
5
. 5
1
K
3 .
6 .
2 6
K