SC2443MLTRT(2007) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2443MLTRT
(Rev.:2007)

Dual-Phase Single or Two Output Synchronous Step-Down Controller

Semtech Corporation 

SC2443

Applications Information (continued)

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 2mA 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 1.4mA 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 2uA

current source and the converter undergoes soft-start.

If overload persists, the SC2443 will undergo repetitive

shutdown and restart.

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(a

consequence of inductor current sense) 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.

Operating Frequency (fs)

The switching frequency in the SC2443 is user-

programmable. The advantages of using constant

frequency operation are simple passive component

selection and ease of feedback compensation. Before

setting the operating frequency, the following trade-offs

should be considered.

1) Passive component size

2) Circuitry efficiency

3) EMI condition

4) Minimum switch on time and

5) Maximum duty ratio

For a given output power, the sizes of the passive

components are inversely proportional to the switching

frequency, whereas MOSFET and Diodes switching losses

are proportional to the operating frequency. Other

issues such as heat dissipation, packaging and the cost

issues are also to be considered. The frequency bands

for signal transmission should be avoided because of EM

interference.

Minimum Switch On Time Consideration

In the SC2443 the falling edge of the clock turns on the

top MOSFET. The inductor current and the sensed voltage

ramp up. After the sensed voltage crosses a threshold

determined by the error amplifier output, the top MOSFET

is turned off. The propagation delay time from the turn-

on of the controlling FET to its turn-off is the minimum

switch on time. The SC2443 has a minimum on time of

about 150ns at room temperature. This is the shortest on

interval of the controlling FET. The controller either does

not turn on the top MOSFET at all or turns it on for at least

150ns.

For a synchronous step-down converter, the operating

duty cycle is VO /VIN . So the required on time for the

top MOSFET is VO /(VIN × FS ) . If the frequency is set

such that the required pulse width is less than 150ns,

then the converter will start skipping cycles. Due to

minimum on time limitation, simultaneously operating at

12

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