Data Sheet

LT83201

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Rev. A 27 of 45

Overcurrent Protection (OCP) and Hiccup Mode

The LT83201 protects against overload and output short-circuit conditions by cycle-by-cycle current limiting both

the current through the top and bottom switches.

Current is sensed in the top switch when it is on. The top switch is immediately turned off when the top switch current

limit (IPEAK-LIMIT) is detected, and the bottom switch is turned on. Current is also sensed in the bottom switch when it

is on, and the top switch is not allowed to turn back on unless the current through the bottom switch has dropped

below the bottom switch current limit (IVALLEY-LIMIT). This effectively stretches the switching period and lowers the

frequency for as long as the protection is required, as the top switch will not be allowed to turn on at the oscillator

clock edge until the bottom switch current drops below IVALLEY-LIMIT. This limits the average current during an output

short-circuit condition to the RMS average of IPEAK-LIMIT and IVALLEY-LIMIT.

The LT83201 recognizes an overcurrent condition when either IPEAK-LIMIT or IVALLEY-LIMIT is triggered, and the VC voltage

rails at its maximum value of VC_CLAMP. Once an overcurrent condition is detected, an internal timer is started. If the

overcurrent condition persists for longer than approximately 1.7ms (tHICC) , then the part enters hiccup mode and

suspends switching for ~12ms (7*tHICC) before soft start is attempted again. Hiccup mode ensures low average power

dissipation under output short-circuit conditions both within the device and the inductor. During this period of

suspended switching in hiccup mode, the VC pin, PG pin, and SET pin are pulled low internally to ensure the part soft-

starts correctly when it next attempts to switch again.

Input Capacitors

The VIN of the LT83201 should be bypassed with at least three ceramic capacitors for the best performance. Two small

ceramic capacitors can be placed close to the part (COPT1, COPT2). These capacitors should be 0402 in size.

Note that a larger input capacitance is required when a lower switching frequency is used. If the input power source

has a high impedance or there is significant inductance due to long wires or cables, additional bulk capacitance may

be necessary. This can be provided with a low-performance electrolytic capacitor.

A ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit.

If the LT83201 is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding

the LT83201’s voltage rating. This situation is easily avoided; for more information, refer to the Application Note 88:

Ceramic Input Capacitors Can Cause Overvoltage Transients.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the

LT83201 to produce the DC output. In this role, it determines the output ripple; thus, a low impedance at the

switching frequency is important. The second function is to store energy to satisfy transient loads and stabilize

LT83201’s control loop. Ceramic capacitors have very low ESR and provide the best ripple performance. For good

starting values, see the Typical Applications section.

Use X5R or X7R types. This choice provides a low output ripple and good transient response. Transient performance

can be improved with a higher-value output capacitor. Increasing output capacitance also decreases the output

voltage ripple. A lower value of the output capacitor is used to save space and cost, but transient performance

suffers, resulting in loop instability. For the suggested capacitor values, see the Typical Applications section.

When choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance

under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a

higher voltage rating may be required.