S-8540 SII [Seiko Instruments Inc], S-8540 Datasheet - Page 13

• Manufacturer Part Number: S-8540
• Description: STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
• Manufacturer: SII [Seiko Instruments Inc]

5. 100% duty cycle
Rev.3.2
4. Current limit circuit
STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
4. 1 V
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
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
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
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
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.
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.
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.
Limit
_00
(current limit setting value) calculated by the above formula. When the voltage difference between
IN
vs. I
peak
3.0
2.5
2.0
1.5
1.0
0.5
0.0
I
Limit
2.5
in the overcurrent state
V
=
IN
(IC: S-8540A33FN, coil: CDRH6D28-100, R
Vsense =
vs. I
peak
Figure 11 l
Limit
Rsense
4.0
(
) is calculated by the following formula:
125
mV)
Seiko Instruments Inc.
peak
5.5
change by input voltage
V
IN
(V)
7.0
SENSE
1.25 A
= 125 mV (typ.) ). The current flowing
S ENSE
8.5
: 100 mΩ)
SENSE
10.0
) between the external coil
S-8540/8541 Series
13