High PFM Threshold

PFM Mode at Light Load

PWM Mode at Heavy Load

Load current increases

Low PFM Threshold

Target Output Voltage

LM8801

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SNVS597H – APRIL 2009 – REVISED MAY 2013

Figure 23. Operation in PFM Mode and Transfer to PWM Mode

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM8801 uses an internal NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

CURRENT LIMITING

A current limit feature allows the LM8801 to protect itself and external components during overload conditions.

PWM mode implements current limit using an internal comparator that trips at 1.1A (typ). If the output is shorted

to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until

the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby

preventing runaway.

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM8801 in shutdown mode. During shutdown the PFET switch,

NFET switch, reference, control and bias circuitry of the LM8801 are turned off. Setting EN high (>1.2V) enables

normal operation. While turning on the device with EN soft-start is activated. EN pin should be set low to turn off

the LM8801 during system power up and under-voltage conditions when the supply is less than 2.3V. Do not

leave the EN pin floating.

SOFT-START

The LM8801 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current

2.3V.

THERMAL SHUTDOWN PROTECTION

The LM8801 has a thermal overload protection function that operates to protect itself from short-term misuse and

overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both

the PFET and the NFET are turned off. When the temperature drops below 130°C, normal operation resumes.

Prolonged operation in thermal overload conditions may damage the device and is considered bad practice.

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Product Folder Links: LM8801