L5981 STMicroelectronics, L5981 Datasheet - Page 12

• Manufacturer Part Number: L5981
• Description: IC REG SW STEP DOWN 1A 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5981

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 1A
• Frequency - Switching: 250kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• Output Voltage: 3 V
• Output Current: 1 A
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 220 KHz to 275 KHz
• Operating Temperature Range: - 40 C to + 150 C
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• Low Dropout Operation: 100 % duty cycle
• For Use With: 497-6385 - BOARD EVAL FOR L5981
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Functional description
4.4
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Overcurrent protection
The L5981 implements the over current protection sensing current flowing through the
power MOSFET. Due to the noise created by the switching activity of the power MOSFET,
the current sensing is disabled during the initial phase of the conduction time. This avoids an
erroneous detection of a fault condition. This interval is generally known as “masking time”
or “blanking time”. The masking time is about 200 ns.
When the over current is detected, two different behaviors are possible depending on the
operating condition.
1.
2.
So the over current protection can be summarized as an “hiccup” intervention when the
output is in regulation and a constant current during the soft-start phase. If the output is
shorted to ground when the output voltage is on regulation, the over current is triggered and
the device starts cycling with a period of 2048 clock cycles between “hiccup” (power
MOSFET off and no current to the load) and “constant current” with very short on-time and
with reduced switching frequency (up to one eighth of normal switching frequency). See
Figure 32.
Output voltage in regulation. When the over current is sensed, the power MOSFET is
switched off and the internal reference (V
error amplifier, is set to zero and kept in this condition for a soft-start time (T
clock cycles). After this time, a new soft-start phase takes place and the internal
reference begins ramping (see
Soft-start phase. If the over current limit is reached the power MOSFET is turned off
implementing the pulse by pulse over current protection. During the soft-start phase,
under over current condition, the device can skip pulses in order to keep the output
current constant and equal to the current limit. If at the end of the "masking time" the
current is higher than the over current threshold, the power MOSFET is turned off and it
will skip one pulse. If, at the next switching on at the end of the "masking time" the
current is still higher than the threshold, the device will skip two pulses. This
mechanism is repeated and the device can skip up to seven pulses. While, if at the end
of the "masking time" the current is lower than the over current threshold, the number of
skipped cycles is decreased of one unit. At the end of soft-start phase the output
voltage is in regulation and if the over current persists the behavior explained above
takes place. (see
for short circuit behavior.
Figure
8.b)
Doc ID 13004 Rev 6
Figure
8.a).
REF
), that biases the non-inverting input of the
SS
, 2048
L5981