SC2453_05 datasheet(16/22 Pages) SEMTECH

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SC2453

POWER MANAGEMENT

Once the power losses P

loss for the top (Pt) and

bottom (P

b) MOSFET’s are known, thermal and package

design at component and system level should be done

to verify that the maximum die junction temperature

j,max, usually 125

C) is not exceeded under the worst-

case condition. The equivalent thermal impedance from

junction to ambient (

ja) should satisfy:

loss

max

−

≤

ja depends on the die to substrate bonding,

packaging material, the thermal contact surface, thermal

compound property, the available effective heat sink

area and the air flow condition (free or forced convection).

Actual temperature measurement of the prototype should

be carried out to verify the thermal design.

Setting the Output Voltage

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

fier is internally tied the 0.5V voltage reference output. 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

converter output voltage. This non-random error is

caused by the feedback voltage divider ratio. It cannot

be corrected by the feedback loop.

The following table lists a few standard resistor combi-

nations for realizing some commonly used output

voltages.

Only the voltages in boldface can be precisely set with

standard 1% resistors.

From this table, one may also observe that when the

value:

o −

−

)

(

)

(

)

(

)

(

and its multiples fall into the standard resistor value

chart (1%, 5% or so), it is possible to use standard value

resistors to exactly set up the required output voltage

value.

The input bias current of the error amplifier also causes

an error in setting the output voltage. The maximum

inverting input bias currents of error amplifiers 1 or 2 is

200nA. Since the non-inverting input is biased to 0.5V,

the percentage error in the second output voltage will be

100%

•(0.2uA) R

01•

/[0.5

• (R

) ].

Valley Current Sensing for Current-Limit

The valley current sensing for current limiting is a unique

scheme which could sense the voltage across the bot-

tom switch MOSFET when it is on. The scheme is robust

with good noise immunity due to reference to ground.

The current sensing point is at a delay time t

dv before

the beginning of a switching cycle. Therefore, the actual

valley current is:

)

(

)

(

−

where, I

VS is the preset valley current limiting threshold.

If a sensed current exceeds the threshold, the top switch

will keep off in the next cycle until the current goes back

below the threshold. In steady state, since the output

voltage is out of regulation in over current condition, the

control loop will try to make maximum duty cycle for the

top switch as it is on, which is usually greater than 80%.

Therefore, as the current falls back below the thresh-

old, it is on in the next almost full cycle. The peak cur-

rent is not controlled and only depends upon circuit pa-

rameters and operating condition in this cycle. The peak

current I

P is:

)

(

)

(

−

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