S-8540 Datasheet, PDF (13/36 Pages) Seiko Instruments Inc – STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.3.2_00
S-8540/8541 Series
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.
The current limit circuit can be enabled by inserting a SENSE resistor (R_SENSE) between the external 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
VOUT pin reaches the current limit detection voltage (V_SENSE = 125 mV (typ.) ). The current flowing
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.
Current limit setting value (I_Limit) is calculated by the following formula:
Vsense ( = 125 mV)
I_Limit =
Rsense
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
I_Limit (current limit setting value) calculated by the above formula. When the voltage difference between
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.
4. 1 V_IN vs. I_peak in the overcurrent state
V_IN vs. I_peak
(IC: S-8540A33FN, coil: CDRH6D28-100, R_SENSE: 100 mΩ)
3.0
2.5
2.0
1.5
1.0
1.25 A
0.5
0.0
2.5
4.0
5.5
7.0
8.5
10.0
V_IN (V)
Figure 11 l_peak change by input voltage
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.
Seiko Instruments Inc.
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