SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Operation (Cont.)
unloaded versions of their respective power phase
voltages) for current sensing. This method does not
require any 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.
Error Amplifiers
In closed loop operation, the error amplifier output ranges
from 1.1V to 3.5V. The upper output operating range of
either error amplifier is reserved for positive current-
sense voltage (CS+ - CS-) and corresponds to positive
(sourcing) output current. If the amplifier swings to its
lower operating range, the amplifier will still modulate
the high-side gate drive duty-ratio. However the peak
current-sense voltage (hence the peak inductor current)
will be limited to a negative value. The error amplifier
output is about 2.2V when the peak sense-voltage is zero.
The built-in offset in the current sense amplifier together
with synchronous continuous-conduction mode of
operation allows the SC2446 to regulate the output
irrespective of the direction of the load current.
The non-inverting input of the first feedback amplifier is
tied to the internal 0.5V voltage reference. Both the non-
inverting and the inverting inputs of the second error
amplifier are brought out as device pins so that the output
of the second converter can be made to track the output
of the first channel. For example in DDR applications,
Channel 1 can be used to generate VDDQ (2.5V) from the
input (5V or 12V) and channel 2 is used to produce a
tracking VTT (1.25V) with VDDQ being its input.
Current-Limit
The maximum current sense voltage of +75mV is the
cycle-by-cycle peak current limit when the load is drawing
current from the converter. There is no cycle-by-cycle
current limiting when the inductor current flows in the
negative direction. However once the valley of the current
sense voltage exceeds –110mV, the corresponding
channel will undergo shutdown and restart (hiccup).
Soft-Start and Overload Protection
The undervoltage lockout circuit discharges the SS/EN
capacitors. After VCC rises above 4.5V, the SS/EN capacitors
are slowly charged by internal 2µA current source. With
internal PNP transistors, the SS/EN voltages clamp the
error amplifier outputs. When the error amplifier output
rises to 2.2V, the high-side MOSFET starts to switch. As
the SS/EN capacitor continues to be charged, 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 then released.
After the SS/EN capacitor is charged above 3.2V (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 or the valley
current-sense voltage exceeds –110mV, an overload latch
will be set and both the top and the bottom MOSFETs 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 0.5V.
The SS/EN capacitor is then recharged with the 2µA current
source and the converter undergoes soft-start. If overload
persists, the SC2446 will undergo repetitive shutdown and
restart (Figure 3).
If the output is short-circuited, the inductor current will
not increase indefinitely between the time the inductor
current reaching its current limit and the instant the
converter shuts down. 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
MOSFETs will be turned off if the SS/EN pin is pulled
below 0.7V.
2004 Semtech Corp.
10
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