Data Sheet

LT83401/LT83402

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Rev. 0

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Overcurrent Protection (OCP)

The LT83401/LT83402 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.

Input Capacitors

The VIN of the LT83401/LT83402 should be bypassed with at least three ceramic capacitors for best performance. Two

small ceramic capacitors can be placed close to the part (CIN1, CIN2). 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 LT83401/LT83402 are plugged into a live supply, the input voltage can ring to twice its nominal value, possibly

exceeding the LT83401/LT83402’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

LT83401/LT83402 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

LT83401/LT83402’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 low output ripple and good transient response. Transient performance

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

The LT83401/LT83402 typically operates at a switching frequency of 2MHz. Table 5 shows some examples of output

capacitors with ideal frequency characteristics when operating at switching frequencies around 2MHz. Figure 51

shows the frequency characteristics of these capacitors. It can be seen that a combination of these capacitors

minimizes the impedance at the switching frequency on the output and keeps the impedance low enough to

suppress any higher-frequency harmonics near the switching frequency, thus achieving the lowest output ripple.