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LT8710

8710f

For more information www.linear.com/LT8710

appenDix

Adequate Load Current

Small value inductors result in increased ripple currents

and thus, due to the limited peak switch current, decrease

the average current that can be provided to the load. In

ordertoprovideadequateloadcurrent,Lshouldbeatleast:

LBOOST ≥

VIN•DC

2• f •

VCSPN

RSENSE1

–

VOUT •IOUT

VIN• η









LDUAL ≥

VIN•DC

2• f •

VCSPN

RSENSE1

–

|VOUT|•IOUT

VIN• η

–IOUT











where:

LBOOST = L1 for boost topologies (see Figure 5)

LDUAL = L1 = L2 for coupled dual inductor topologies

(see Figures 6 and 7)

LDUAL = L1 || L2 for uncoupled dual inductor topolo-

gies (see Figures 6 and 7)

= Switch duty cycle (see previous section)

VCSPN = Current limit voltage at the operating switch

duty cycle (see Max Current Limit vs Duty

Cycle (CSP – CSN) plot in the Typical Per-

formance Characteristics)

RSENSE1 = Current sense resistor connected across

the CSP-CSN pins (see Block Diagram)

= Power conversion efficiency (assume 90%)

= Switching frequency

IOUT

= Maximum output current

Negative values of LBOOST or LDUAL indicate that the out-

put load current, IOUT, exceeds the switch current limit

capability of the converter. Decrease RSENSE1 to increase

the switch current limit.

Avoiding Subharmonic Oscillations

The LT8710’s internal slope compensation circuit will

prevent subharmonic oscillations that can occur when

the duty cycle is greater than 50%, provided that the in-

ductance exceeds a minimum value. In applications that

operate with duty cycles greater than 50%, the inductance

must be at least:

LMIN≥

VIN•RSENSE1•(2•DC–1)

40m•DC• f •(1–DC)

where

LMIN = L1 for boost topologies (see Figure 5)

LMIN = L1 = L2 for coupled dual inductor topologies

(see Figures 6 and 7)

LMIN = L1 || L2 for uncoupled dual inductor topologies

(see Figures 6 and 7)

Maximum Inductance

Excessive inductance can reduce ripple current to levels

thataredifficultforthecurrentcomparator(A5intheBlock

Diagram) to cleanly discriminate, thus causing duty cycle

jitter and/or poor regulation. The maximum inductance

can be calculated by:

LMAX ≤

VIN•RSENSE1•DC

5m• f

where:

LMAX = L1 for boost topologies (see Figure 5)

LMAX = L1 = L2 for coupled dual inductor topologies

(see Figures 6 and 7)

LMAX = L1 || L2 for uncoupled dual inductor topologies

(see Figures 6 and 7)

Inductor Current Rating

The inductor(s) must have a rating greater than its (their)

peak operating current to prevent inductor saturation,

which would result in efficiency losses. The maximum

Boost

Topology

SEPIC

Inverting

Topologies

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