SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
RsCs
=
L
R eq
,
for Rs and RS3.
ILMR eq
+
Rs
Rs3
VO
=
75mV,
Rs2 is then obtained from
Rs2
=
Rs3Rs
Rs3 − Rs
.
If the current limit is to be set to ILM = 5A with the existing
power circuit parameter and Cs, it is calculated that
Rs=4.12 kΩ, Rs3=190 kΩ and Rs2=4.22 kΩ.
Similar steps and equations apply to the current limit
setting and scaling for current sinking mode.
Remark 4: When the current limit ILM is lower than ILMcp,
the designer has the freedom of selecting higher Rds(ON)
MOSFETs to reduce cost. As a result, Reg is increased
and ILMcp is reduced. Although the use of low-cost
MOSFET’s is always preferred, the current-limit setting
technique described above allows quick adjustment on a
well-tested prototype without the need to replace the
power MOSFETs.
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 (75% for Ch1, 72%
for Ch2) of the reference voltage, the controller will shut
off both the top and the bottom MOSFET’s. Meanwhile
an internal 1.4µA current source discharges the soft-start
capacitor C32(C33) connected to the SS/EN pin.
When the capacitor is discharged to 0.5V, a 2µA 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.
a) The time taken to discharge the capacitor from 3.2V
to 0.5V
t ssf
=
C32
(3.2 − 0.5)V .
1.4µA
If C32 = 0.1µF, tssf is calculated as 193ms.
b) The soft start time from 0.5V to 3.2V
t ssr
=
C32
(3.2 − 0.5)V
2µA
.
When C32 = 0.1µF, tssr is calculated as 135ms. 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
2µA
.
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.
2004 Semtech Corp.
21
www.semtech.com
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