SC2620 datasheet(9/26 Pages) SEMTECH

9 / 26 page

 2006 Semtech Corp.

www.semtech.com

SC2620

POWER MANAGEMENT

Applications Information

Setting the Output Voltage

The regulator output voltage is set with an external resistive

divider (Figure 4) with its center tap tied to the FB pin.

)

(

OUT

−

(1)

The percentage error due the input bias current of the

error amplifier is

)

(

100

OUT



⋅

−

∆

Example: Determine the output voltage error of a

OUT =

converter with

Ω

From (1),

Ω

−

⋅

Ω

205

)

(

061

)

205

(

100

OUT

−



⋅

−

∆

This error is at least an order of magnitude lower than the

ratio tolerance resulting from the use of 1% resistors in the

divider string.

Setting the Channel Frequency

The switching frequency of the master oscillator is set with

an external resistor from the ROSC pin to ground. Channel

frequency is one-half of that of the master oscillator. A

graph of channel frequency against R

OSC is shown in the

“Typical Performance Characteristics”. Channel frequency

is programmable up to 1.4MHz.

Channel switching frequency is limited by the minimum

controllable on time at low duty cycles. For V

IN > 20V, setting

the switching frequency below 500kHz makes converter

output short circuit operation more robust. These will be

described in more details later.

Minimum On Time Consideration

The operating duty cycle of a non-synchronous step-down

switching regulator in continuous-conduction mode (CCM)

is given by

CESAT

OUT

−

(2)

where V

CESAT is the switch saturation voltage and VD is

voltage drop across the rectifying diode.

Duty cycle decreases with increasing

OUT

ratio. In peak

current-mode control, the PWM modulating ramp is the

sensed current ramp of the power switch. This current

ramp is absent unless the switch is turned on. The

intersection of this ramp with the output of the voltage

feedback error amplifier determines the switch pulse width.

The propagation delay time required to immediately turn

off the switch after it is turned on is the minimum

controllable switch on time (T

ON(MIN)). Closed-loop

measurement of the SC2620 with low

OUT

ratios shows

VOUT

15nA

SC2620

Figure 4. V

OUT is set with a Resistive Divider

Minimum On Time

vs Ambient Temperature

100

110

120

130

-50

-25

100

Temperature (°C)

Figure 5. Variation of Minimum On Time with

Ambient Temperature.

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