SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441ITSTRT
Semtech Corporation
SC2441ITSTRT Datasheet PDF : 35 Pages
| |||
SC2441
POWER MANAGEMENT
Operation (Cont.)
Although accurate and far easier to lay out than high-
side resistor sensing, a pair of precision sense resistors
adds cost to the converter. The SC2441 has provision to
reconstruct a differential voltage proportional to the
inductor current at the output of the converter. The
voltage to current ratio or the equivalent sense resistance
Req is a combination of high-side and low-side MOSFET
RDS(ON) ’s and the inductor series resistance (hence the
name “combi-sense”). The SC2441 provides the virtual
phase voltages VPN1 and VPN2 (these are unloaded
versions of their respective phase voltages) for current
sensing. This method does not require precision sense
resistor. It is cheaper to implement but is less accurate
than resistor current sensing. Since the sensed voltage
is developed at the output of the step-down converter, it
is less prone to switching transient spikes. This method
will be described in more details in the Applications
Information section.
Boost switch current is sensed with an integrated sense
resistor with a current-limit of 1.7A.
Error Amplifiers
All error amplifiers in the SC2441 are transconductance
amplifiers. Converters are compensated with series RC
network from the COMP pins to the ground. An additional
small parallel capacitor may be required for stability.
In closed loop operation, the step-down error amplifiers
output range from 1.7V to 3.5V. There is no control (high-
side) gate drive until the COMP voltage exceeds 1.6V.
Both non-inverting inputs of the feedback amplifiers are
tied to an internal 0.5V voltage reference.
The error amplifier of the step-up converter has 1.25V
as its reference voltage. Its output voltage ranges from
0.8V to 1.35V in closed-loop operation.
Current-Limit
The maximum current sense voltage of +50mV is the
cycle-by-cycle peak current limit when the load is drawing
current from the converter.
Soft-Start and Overload Protection
The undervoltage lockout circuit discharges the SS/EN
capacitors. After VCC rises above 4.65V, the SS/EN
capacitors are slowly charged by internal 2.3µA current
sources. With internal PNP transistors, the SS/EN voltages
clamp the error amplifier outputs. When the error amplifier
output rises to 1.7V, the high-side MOSFET starts to switch.
As the SS/EN capacitor continues to charge, the COMP
voltage follows. The converter gradually delivers increasing
power to the output. The inductor current follows the COMP
voltage envelope until the output goes into regulation. The
SS/EN clamp on COMP is released.
After the SS/EN capacitor is charged above 3.25V (high
enough for the error amplifier to provide full load current),
the overload detection circuit is activated. If the output
voltage falls below 70% of its set value, an overload latch
will be set and both the top and the bottom MOSFET’s
will be turned off. The SS/EN capacitor is slowly
discharged with an internal 1.4µA current sink. The
overload latch is reset when the SS/EN capacitor is
discharged below 0.47V. The SS/EN capacitor is then
recharged with the 2.3µA current source and the converter
undergoes soft-start. If overload persists, the step-down
converters will undergo repetitive shutdown and restart
(hiccup). Soft-start process should be slow enough to allow
the output to reach 70% of its final value before the SS/
EN capacitor is charged above 3.25V (see Figure 4).
If the output is short-circuited, the inductor current will
not increase indefinitely between the time the inductor
current reaching its current limit and shutdown. This is
due to cycle skipping reduces the actual operating
frequency.
The SS/EN pin can also be used as the enable input for
that channel. Both the high-side and the low-side
MOSFET’s will be turned off if the SS/EN pin is pulled
below 0.5V.
2005 Semtech Corp.
12
www.semtech.com
Share Link: 