SC2446AEVB datasheet(16/27 Pages) SEMTECH

16 / 27 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446A

Note that R

S2 is selected as RS//RS1 in order to reduce

the bias current effect of the current amplifier in

SC2446A.

b) When the required current limit I

LM is less than ILMcp,

one just needs to remove R

s1 and solve

for R

s and RS3.

s2 is then obtained from

−

Similar steps and equations apply to the current limit

setting and scaling for current sinking mode.

Overload Protection and Hiccup

During start-up, the capacitor from the SS/EN pin to

ground functions as a soft-start capacitor. After the

converter starts and enters regulation, the same

capacitor operates as an overload shutoff timing

capacitor. As the load current increases, the cycle-by-

cycle current-limit comparator will first limit the inductor

current. Further increase in loading will cause the output

voltage (hence the feedback voltage) to fall. If the

feedback voltage falls to less than (50% for Ch1, 50%

for Ch2) of the reference voltage, the controller will shut

off both the top and the bottom MOSFETs. Meanwhile

an internal net 1.2

µA current source discharges the soft-

start capacitor C

32(C33) connected to the SS/EN pin.

When the capacitor is discharged to 0.5V, a 1.8

µA current

source recharges the SS/EN capacitor and converter

restarts. If overload persists, the controller will shut down

the converter when the soft start capacitor voltage

exceeds 3.2V. The converter will repeatedly start and shut

off until it is no longer overloaded. This hiccup mode of

overload protection is a form of foldback current limiting.

The following calculations estimate the average inductor

current when the converter output is shorted to the

ground.

a) The time taken to discharge the capacitor from 3.2V

to 0.5V

)

(

ssf

−

If C

32 = 0.1µF, tssf is calculated as 225ms.

b) The soft start time from 0.5V to 3.2V

)

(

ssr

−

When C

32 = 0.1µF, tssr is calculated as 150ms. Note that

during soft start, the converter only starts switching when

the voltage at SS/EN exceeds 1.2V.

c) The effective start-up time is

)

(

sso

−

The average inductor current is then

ssr

ssf

sso

LMcp

Leff

≈ 0.30 I

LMcp and is independent of the soft start

capacitor value. The converter will not overheat in hiccup.

Setting the Output Voltage

The non-inverting input of the channel-one error amplifier

is internally tied the 0.5V voltage reference output (Pin

8). The non-inverting input of the channel-two error

amplifier is brought out as a device pin (Pin 10) to which

the user can connect Pin 8 or an external voltage

reference. A simple voltage divider (R

o1 at top and Ro2 at

bottom) sets the converter output voltage. The voltage

feedback gain h=0.5/V

o is related to the divider resistors

value as

−

Once either R

o1 or Ro2 is chosen, the other can be

calculated for the desired output voltage V

o. Since the

number of standard resistance values is limited, the

calculated resistance may not be available as a standard

value resistor. As a result, there will be a set error in the

Application Information (Cont.)

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