sc2441a Semtech Corporation, sc2441a Datasheet - Page 19

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

In closed loop operation, EA1 and EA2 output voltage
vary from 1.2V to 3.5V with the range 1.2V to 1.85V
corresponding to negative peak sense voltages. Both gate
drives are kept off until the COMP voltage exceeds 1.85V
in start up.
The error amplifier of the step-up converter has a 1.25V
reference voltage. Its output voltage excursion is from 0.8V
to 1V in closed-loop operation.
Current-Limit
The 25mV maximum current sense voltage is the cycle-by-
cycle peak current limit of the step-down controller.
Gate Drivers
The SC2441A uses an adaptive non-overlapping control
scheme to switch the upper and the synchronous
MOSFETs. The synchronous MOSFET of each step-down
channel is turned off at the falling-edge of the phase
clock. The control (upper) MOSFET is not turned on until
the synchronous gate drive goes low. The phase inductor
current ramps up. When the sensed inductor current
reaches the threshold determined by the error amplifier
output and ramp compensation, the control MOSFET is
turned off. The synchronous MOSFET is not turned on
until the upper gate drive goes low.
The supply voltage for the upper gate driver is obtained
from a diode-capacitor bootstrap circuit. If the bootstrap
capacitor is charged from V
voltage will swing from approximately 2V
synchronous gate drive swings from V
Soft-Start and Overload Protection
Figure 4 shows the functional diagram of the soft-start
and overload protection circuit. The soft-start capacitor
C
Together they set the soft-start time. Before V
4.5V, the undervoltage lockout circuit discharges C
ground. After V
slowly charged by R
disable both the upper and lower gate drives. Hysteretic
comparator C
POWER MANAGEMENT
Operation (Cont.)
SS
2006 Semtech Corp.
and its charging resistor R
1
resets the latch L
CC
rises above 4.5V, Q
SS
. Comparator C
CC
, then the high-side gate drive
SS
1
are tied to the SS/EN pin.
so that hiccup is disabled
1
CC
turns off and C
2
to ground.
and latch L
CC
to ground. The
CC
rises to
2
SS
SS
first
to
is
19
during start up. As mentioned above, there is no PWM
(=TGON) pulse until C
corresponding COMP rises above 1.85V. Once the first
TGON pulse appears, L
that channel are enabled.
After C
Hiccup is armed. If the output voltage is less than 70%
of the set value due to improper start up or output
overload, then C
drivers of the channel are turned off and the 10 A
current source discharges C
to ensure full discharge of C
process should be slow enough to allow the output to
reach 70% of its final value before hiccup is armed. The
overload latch L
discharged below 0.5V. The 10 A current source turns
off. C
undergoes soft-start. If overload persists, the step-down
converters will undergo repetitive shutdown and restart
(hiccup).
If the output is short-circuited, the inductor current will
not increase indefinitely between the times the inductor
current reaching its current limit and shutdown. This is
due to cycle skipping reduces the actual operating
frequency.
Pulling the SS/EN pin below 0.8V with an open-collector
transistor sets the disable latch L
drives. The SS/EN pin can be used as the enable input for
the controller.
The soft start timing diagram and the hiccup operation
timing diagram are shown in Figures 6a and 6b
respectively.
SS
SS
capacitor is recharged by R
is charged above 3.3V, C
3
will set the overload latch L
1
is reset when the C
2
is reset and both gate drivers of
SS
SS
is above 1.25V and the
SS
. R
down to 0.5V. Soft-start
SS
2
must be large enough
and turns off the gate
SS
1
and the converter
output goes low.
SC2441A
SS
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capacitor is
1
. Both gate