STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

S-8540/8541 Series

13

4. Current limit circuit

The S-8540/8541 series contains a current limit circuit.

The current limit circuit is designed to prevent thermal destruction of external transistors due to overload

or magnetic saturation of the coil.

and the output pin VOUT, and connecting the node for the SENSE resistor and the coil to the SENSE

pin.

A current limit comparator in the IC is used to check whether the voltage between the SENSE pin and

through the external transistor is limited by turning it off during the left time of the oscillation period after

detection. The transistor is turned on again at the next clock and current limit detection resumes. If

the overcurrent state still persists, the current limit circuit operates again, and the process is repeated.

If the overcurrent state is eliminated, the normal operation resumes. Slight overshoot occurs in the

output voltage when the overcurrent state is eliminated.

Rsense

mV)

125

(

Vsense

=

If the change with time of the current flowing through the sense resistor is higher than the response

speed of the current limit comparator in the IC, the actual current limit value becomes higher than the

VIN pin and VOUT pin is large, the actual current limit value increases since the change with time of the

current flowing through the sense resistor becomes large.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

2.5

4.0

5.5

7.0

8.5

10.0

Ω)

1.25 A

When the output voltage is approximate 1.0 V or less, the load short-circuit protection does not work,

since the current limit circuit does not operate.

When the current limit circuit is not used, remove the SENSE resistor and connect the SENSE pin to the

VSS or VOUT pin.

5. 100% duty cycle

The S-8540/8541 series operates up to the maximum duty cycle of 100%. The switching transistor is

kept on continuously to supply current to the load, when the input voltage falls below the preset output

voltage value. The output voltage in this case is equal to the subtraction of lowering causes by DC

resistance of the coil and on resistance of the switching FET from the input voltage.

Even when the duty cycle is 100%, the current limit circuit works when overcurrent flows.