SC2446AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2446AEVB Datasheet PDF : 27 Pages
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SC2446A
POWER MANAGEMENT
Application Information (Cont.)
Note that RS2 is selected as RS//RS1 in order to reduce
the bias current effect of the current amplifier in
SC2446A.
b) When the required current limit ILM is less than ILMcp,
one just needs to remove Rs1 and solve
RsCs
=
L
RL
,
a) The time taken to discharge the capacitor from 3.2V
to 0.5V
t ssf
= C32
(3.2 − 0.5)V
1.2µA
.
If C32 = 0.1µF, tssf is calculated as 225ms.
b) The soft start time from 0.5V to 3.2V
t ssr
=
C32
(3.2 − 0.5)V
1.8µA
.
for Rs and RS3.
ILMRL
+
Rs
Rs3
VO
=
50mV,
R is then obtained from
s2
Rs2
=
Rs3Rs
Rs3 − Rs
.
When C32 = 0.1µF, tssr is calculated as 150ms. Note that
during soft start, the converter only starts switching when
the voltage at SS/EN exceeds 1.2V.
c) The effective start-up time is
t sso
=
C32
(3.2 − 1.2)V
1.8µA
.
Similar steps and equations apply to the current limit
setting and scaling for current sinking mode.
Overload Protection and Hiccup
During start-up, the capacitor from the SS/EN pin to
ground functions as a soft-start capacitor. After the
converter starts and enters regulation, the same
capacitor operates as an overload shutoff timing
capacitor. As the load current increases, the cycle-by-
cycle current-limit comparator will first limit the inductor
current. Further increase in loading will cause the output
voltage (hence the feedback voltage) to fall. If the
feedback voltage falls to less than (50% for Ch1, 50%
for Ch2) of the reference voltage, the controller will shut
off both the top and the bottom MOSFETs. Meanwhile
an internal net 1.2µA current source discharges the soft-
start capacitor C32(C33) connected to the SS/EN pin.
When the capacitor is discharged to 0.5V, a 1.8µA current
source recharges the SS/EN capacitor and converter
restarts. If overload persists, the controller will shut down
the converter when the soft start capacitor voltage
exceeds 3.2V. The converter will repeatedly start and shut
off until it is no longer overloaded. This hiccup mode of
overload protection is a form of foldback current limiting.
The following calculations estimate the average inductor
current when the converter output is shorted to the
ground.
The average inductor current is then
ILeff
= ILMcp
tsso .
tssf + tssr
ILeff ≈ 0.30 ILMcp and is independent of the soft start
capacitor value. The converter will not overheat in hiccup.
Setting the Output Voltage
The non-inverting input of the channel-one error amplifier
is internally tied the 0.5V voltage reference output (Pin
8). The non-inverting input of the channel-two error
amplifier is brought out as a device pin (Pin 10) to which
the user can connect Pin 8 or an external voltage
reference. A simple voltage divider (R at top and R at
o1
o2
bottom) sets the converter output voltage. The voltage
feedback gain h=0.5/V is related to the divider resistors
o
value as
Ro2
=
1
h
−
h
Ro1.
Once either R or R is chosen, the other can be
o1
o2
calculated for the desired output voltage V . Since the
o
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
2005 Semtech Corp.
16
www.semtech.com
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