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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

where VOUTx is the output voltage for the corresponding

channel, VBOTx is the voltage across the bottom switch for

the corresponding channel, fSW is the switching frequency

in MHz, and L is in μH.

Once the inductance is selected, the inductor current

ripple and peak current can be calculated as:

∆ILx =

VOUTx

Lx • fSW

• 1–

VOUTx

PVINx(MAX)

















ILx(PEAK) = IOUTx(MAX) +

∆ILx

where PVINx(MAX) is the maximum input voltage for each

channel in a given application and IOUTx(MAX) is the maxi-

mum output current for each channel in a given applica-

tion. To avoid overheating and poor efficiency, an induc-

tor must be chosen with an RMS current rating that is

greater than the maximum expected output load of the

application. In addition, the saturation current rating of the

inductor must be higher than the load plus half the ripple

current. Finally, for best efficiency the inductor series

resistance should be as small as possible, and the core

material should be intended for the application switching

frequency.

The optimum inductor for a given application may differ

from the one indicated by this design guide. Careful evalu-

ation of the application circuit should be completed with

the chosen inductor to ensure adequate design margin.

Buck: Shorted Output Protection

If the bottom MOSFET current exceeds the valley current

limit at the start of a clock cycle, the top MOSFET is kept

off until the overcurrent situation clears. This prevents the

buildup of inductor current during a shorted output condi-

tion. Further, during overload or short-circuit conditions,

the LT8603 safely tolerates operation with a saturated

inductor.

Buck: Input Capacitor Selection

Step-down converters draw current from the input sup-

ply in pulses with very fast rise and fall times. An input

capacitor is required to reduce the resultant voltage ripple

at the input and minimize EMI. For this function, a ceramic

X7R or X5R bypass capacitor should be placed between

each buck channel’s PVIN pin and ground. To be most

effective, the input capacitor must have low impedance

at the switching frequency and an adequate ripple current

rating.

The worst case ripple current occurs when VOUT is one-

half PVIN. Under this condition, the ripple current is:

ICIN(RMS) =

IOUT

Reasonable starting values for the input capacitor are:

4.7μF for Channels 1 and 3

10μF for Channel 2

A word of caution is in order regarding the use of ceramic

capacitors at the input. A ceramic input capacitor can

combine with stray inductance to form a resonant tank

circuit back to the supply. If power is applied quickly (for

example by plugging the circuit into a live power source),

this tank can ring, as much as doubling the input volt-

age. The solution is to either clamp the input voltage or

dampen the tank circuit by adding a lossy capacitor in

parallel with the ceramic capacitor. For details see Analog

Devices Application Note 88.

Buck: Output Capacitor Selection

The output capacitor performs two functions. First, it fil-

ters the inductor current to generate an output with low

voltage ripple. Second, it stores energy to minimize droop

and overshoot during transient loads. Because the LT8603

buck converters are able to operate at a high frequency,

minimal output capacitance is necessary. The internally

compensated current mode control loops are stable with-

out requiring a minimum series resistance (ESR) in the

output capacitor. Therefore ceramic capacitors may be

used and will result in very low output ripple.

You can estimate output ripple with the following equa-

tions as appropriate:

VRIPPLE =

∆IL

8 • fSW •COUT

,for ceramic

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