Data Sheet

LT8615

analog.com

Rev 0

21 of 31

∆IL =

VOUT

L × fSW

(1 −

VOUT

VIN(MAX)

)

where, fSWis the switching frequency of the LT8615, and L is the value of the inductor. Therefore, the maximum output

current the LT8615 delivers depends on minimum the switch current limit, the inductor value, and the input and

output voltages. The inductor value may have to be increased if the inductor ripple current does not allow sufficient

maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired

application.

The optimum inductor for a given application may differ from the one indicated by this design guide. A larger value

inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring

smaller load currents, the value of the inductor may be lower and the LT8615 may operate with higher ripple current.

This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that

low inductance may result in discontinuous mode operation, which further reduces maximum load current.

For more information about maximum output current and discontinuous operation, refer to Application Note 44:

LT1074/LT1076 Design Manual.

For duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic

oscillation. Refer to Application Note 19: LT1070 Design Manual for more details.

LMIN =

VIN × (2 × DC – 1)

1.5 × fSW

where, DC is the duty cycle ratio (VOUT/VIN) and fSW is the switching frequency.

Input Capacitor

Bypass the input of the LT8615 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance

over temperature and applied voltage, and should not be used. A 4.7μF to 10μF ceramic capacitor is adequate to

bypass the LT8615 and easily handles the ripple current. Note that larger input capacitance is required when a lower

switching frequency is used. If the input power source has 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.

Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple at the LT8615 and to force this very high frequency

switching current into a tight local loop, minimizing the EMI. A 4.7μF capacitor is capable of this task, but only if it is

placed close to the LT8615 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor

concerns the maximum input voltage rating of the LT8615. A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank circuit. If the LT8615 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly exceeding the LT8615’s voltage rating. This situation is

easily avoided (refer to 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

LT8615 to produce the DC output. In this role it determines the output ripple; thus, low impedance at the switching

frequency is important. The second function is to store energy to satisfy transient loads and stabilize the LT8615’s

control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple

performance. A good starting value is: