AN4027 datasheet(8/41 Pages) STMICROELECTRONICS

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Main characteristics and circuit description

AN4027

8/41

DocID022604 Rev 3

Figure 2. Burst-mode circuit block diagram

The output current is sensed by a resistor (RCS); the voltage drop across this resistor is

amplified by the TSC101, a dedicated high-side current sense amplifier; its output is

compared to a set reference by the TSM1014; if the output load is high, the signal fed into

the CC- pin is above the reference voltage, CC_OUT stays down and the optocoupler

transistor pulls up the L6699 STBY pin to the RFMIN voltage (2 V), setting continuous

switching operation (no burst mode); if the load decreases, the voltage on CC- falls below

the set threshold, CC_OUT goes high opening the connection between RFMIN and STBY

and allowing burst mode operation by the L6699. RCS is dimensioned considering two

constraints. The first is the maximum power dissipation allowed, based on the efficiency

target. The second limitation is imposed by the need to feed a reasonable voltage signal into

the TSM1014A inverting input. In fact, signals which are too small would affect system

accuracy.

On this board, the maximum acceptable power dissipation has been set to

Ploss,MAX = 500 mW. RCS maximum value is calculated as follows:

Equation 1

The burst mode threshold is set at 18 W corresponding to IBM = 1.5 A output current at 12 V.

Choosing VCC+min = 300 mV as minimum reference of the TSM1014A, which permits

a good signal to noise ratio, the RCS minimum value is calculated as follows:

Equation 2

The actual value of the mounted resistor is 2 m

, corresponding to Ploss = 312 mW power

losses at full load. The actual resistor value at the burst mode threshold current provides an

output voltage by the TSC101 of 83 mV. The reference voltage of TSM1014 Vcc+ must be

set at this level. The resistor divider setting the TSM1014 threshold RH and RL should be in

the range of k

 to minimize dissipation. Selecting RL = 22 K, the right RH value is obtained

as follows:

3.2mΩ

MAX

out,

MAX

loss,

MAX

CS,

RCS min



VCC min

100 CBM



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

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