AN2683 datasheet(10/38 Pages) STMICROELECTRONICS

10 / 38 page

Circuit description

AN2683

10/38

for output 1:

Equation 9

for output 2:

Equation 10

These values conform to the specification. They are higher in a practical circuit because of

parasitic inductance and loop resistance. Good circuit board layout techniques are

essential. Additionally, because of the constant on-time control, the system regulates the

output voltage by the valley value of the ripple voltage. A minimum amount of ripple voltage

of 30 mV should be on the comp pin to accomplish this. Since the calculated ripple voltage

is much lower than this, an additional circuit called the virtual ESR network is incorporated

to provide the additional voltage. Before addressing this design, the current limit resistor

values will be established. In this design the RDS(on) of the lower MOSFETS is used to

implement the current limit. For output 1 with its relatively low output current the MOSFET

chosen was the STS8DNF3LL with a nominal RDS(on) of 18 mΩ. This particular part is a

dual, that is two MOSFETs are contained in the same SO-8 package realizing further circuit

board space savings. The current limit is a valley type that operates during the conduction of

the low side MOSFET. A 100

µA internal current generator connected to the Csense pin

along with a resistor establishes a voltage to which the voltage generated by the RDS(on) is

compared. If the RDS(on) voltage is greater, then the voltage at the Csense pin the generation

of a new conduction cycle is inhibited. The value of Rcsense is determined by:

Equation 11:

The 18 m

Ω value for RDS(on) is a nominal 25 °C number. As current is switched through the

device and the ambient is raised, the RDS(on) increases. An increase of approximately

140% is used. Targeting the maximum output current (Ioutmax) at 3.375 A and having a Ir of

0.750 A the valley current value is:

0.75A

Ω 1.5mV

⋅

3.15A

545

µΩ 1.9mV

⋅

sense

DS on

()

valley

⋅

100

µA

------------------------------------------

AN2683 Datasheet(PDF) 10 Page - STMicroelectronics

AN2683 Datasheet(HTML) 10 Page - STMicroelectronics

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 27 28 29 30 31 32 33 34 35 36 37 38

Datasheet Download

Link URL