SC2441ITSTRT datasheet(12/35 Pages) SEMTECH

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 2005 Semtech Corp.

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POWER MANAGEMENT

SC2441

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

eq is a combination of high-side and low-side MOSFET

DS(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 V

CC rises above 4.65V, the SS/EN

capacitors are slowly charged by internal 2.3

µA 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 1.4

µA 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 2.3

µA 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.

Operation (Cont.)

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