SC2620 datasheet(8/26 Pages) SEMTECH

8 / 26 page

 2006 Semtech Corp.

www.semtech.com

SC2620

POWER MANAGEMENT

Operation

The SC2620 is a 30V 2-channel constant-frequency peak

current-mode step-down switching regulator with integrated

2.3A power transistors. Both regulators in the SC2620

operate from a common input power supply and share the

same voltage reference and the master oscillator. Turn-on

of the power transistors is phase-shifted by 180

°. The two

regulator cores are otherwise completely identical,

independent and are capable of producing two separate

outputs from the same input.

The channel frequency can be programmed with an

external resistor from the ROSC pin to ground. This allows

the designer to set the switching frequency according to

the input to the output voltage conversion ratio.

Peak current-mode control is utilized for the SC2620. The

double reactive poles of the output LC filter are reduced to

a single real pole by the inner current loop, easing loop

compensation. Fast transient response can be achieved

with a simple Type-2 compensation network. Switch

collector current is sensed with an integrated 6.3m

Ω sense

resistor. The sensed current is summed with slope-

compensating ramp before it is compared with the

transconductance error amplifier output. The PWM

comparator tripping point determines the switch turn-on

pulse width (Figure 2). The current-limit comparator ILIM

turns off the power switch when the sensed-signal exceeds

the 20mV current-limit threshold. ILIM therefore provides

cycle-by-cycle limit. Current-limit does not vary with duty-

cycle.

An external charge pump (formed by the capacitor C

2 and

the diode D

3 in Figure 1(a)) generates a voltage higher

than the input rail at the BOOST pin. The bootstrapped

voltage generated becomes the supply voltage for the

power transistor driver. Driving the base of the power

transistor above the input power supply rail minimizes the

power transistor turn-on voltage and maximizes efficiency.

The SS pin is a multiple-function pin. An external capacitor

connected from the SS pin to ground together with the

internal 1.8µA and 2.6µA current sources set the soft-

start and overload shutoff times of the regulator (Figure

3). The SS pin can also be used to shut off the corresponding

regulator. When either SS pin is pulled below 0.8V, that

regulator is turned off. If both SS pins are pulled below

0.2V, then the SC2620 undergoes overall shutdown. The

current drawn from the input power supply reduces to

40µA. When either SS pin is released, the corresponding

soft-start capacitor is charged with a 2µA current source

(not shown in Figure 3). As either SS voltage exceeds 0.3V,

the internal bias circuit of the SC2620 is enabled. The

SC2620 draws 3.5mA from V

IN. An internal fast charge

circuit quickly charges the soft-start capacitor to 1V. At

this juncture, the fast charge circuit turns off and the 1.8µA

current source slowly charges the soft-start capacitor. The

output of the error amplifier is forced to track the slow

soft-start ramp at the SS pin. When the COMP voltage

exceeds 1.1V, the switching regulator starts to switch.

During soft-start, the current limit of the converter is

gradually increased until the converter output comes into

regulation.

Hiccup overload protection is utilized in the SC2620.

Overload shutdown is disabled during soft-start (V

SS < 2V).

In Figure 3 the reset input of the overload latch will remain

high if the SS voltage is below 2V. Once the soft-start

capacitor is charged above 2V, the overload shutdown latch

is enabled. As the load draws more current from the

regulator, the current-limit comparator will limit the peak

inductor current. This is cycle-by-cycle current limiting.

Further increase in load current will cause the output

voltage to decrease. If the output voltage falls below 70%

of its set point, then the overload latch will be set and the

soft-start capacitor will be discharged with a net current

of 0.8µA. The switching regulator is shut off until the soft-

start capacitor is discharged below 1V. At this moment,

the overload latch is reset. The soft-start capacitor is

recharged and the converter again undergoes soft-start.

The regulator will go through soft-start, overload shutdown

and restart until it is no longer overloaded.

The power good comparator indicates that the channel 1

regulator output has risen to within 10% of its set value.

The open collector output of the power good comparator

will be actively pulled low if its feedback voltage is below

0.9V.

SC2620 датащи(PDF) 8 Page - Semtech Corporation

SC2620 датащи(HTML) 8 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

датащи скачать

ссылки URL