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LT8580

Boost/SEPIC/Inverting DC/DC Converter with 1A, 65V Switch, Soft-Start and Synchronization

Linear Technology 

LT8580

Applications Information

provided to a load (IOUT). In order to provide adequate

load current, L should be at least:

LBOOST

>

DC • VIN

2(f)

⎛

⎝⎜ILIM

−

|VOUT|

VIN

• IOUT

•h

⎞

⎟

⎠

for boost, topologies, or:

LDUAL

>

DC • VIN

⎛

2(f) ⎜ILIM−

⎝

VOUT

VIN

• IOUT

•h

⎞

−

IOUT

⎟

⎠

for the SEPIC and inverting topologies.

where:

LBOOST = L1 for boost topologies (see Figure 15)

LDUAL = L1 = L2 for coupled dual inductor topologies

(see Figure 16 and Figure 17)

LDUAL = L1||L2 for uncoupled dual inductor topologies

(see Figure 16 and Figure 17)

DC = switch duty cycle (see previous section)

ILIM = switch current limit, typically about 1.2A at 50%

duty cycle (see the Typical Performance Characteristics

section).

h = power conversion efficiency (typically 85% for boost

and 83% for dual inductor topologies at high currents).

f = switching frequency

IOUT = maximum load current

Negative values of L indicate that the output load current

IOUT exceeds the switch current limit capability of the

LT8580.

Avoiding Subharmonic Oscillations: The LT8580’s internal

slope compensation circuit can prevent subharmonic oscil-

lations that can occur when the duty cycle is greater than

50%, provided that the inductance exceeds a minimum

value. In applications that operate with duty cycles greater

than 50%, the inductance must be at least:

LMIN

>

VIN

1.25 • (DC − 300nS

•

f)

•

f



2  DC − 1

1− DC



LMIN = L1 for boost topologies (see Figure 15)

LMIN = L1 = L2 for coupled dual inductor topologies

(see Figure 16 and Figure 17)

LMIN = L1||L2 for uncoupled dual inductor topologies

(see Figure 16 and Figure 17)

Maximum Inductance: Excessive inductance can reduce

current ripple to levels that are difficult for the current com-

parator (A3 in the Block Diagram) to cleanly discriminate,

thus causing duty cycle jitter and/or poor regulation. The

maximum inductance can be calculated by:

LMAX

=

VIN − VCESAT

IMIN-RIPPLE

•

DC

f

where

LMIN = L1 for boost topologies (see Figure 15)

LMIN = L1 = L2 for coupled dual inductor topologies

(see Figure 16 and Figure 17)

LMIN = L1||L2 for uncoupled dual inductor topologies

(see Figure 16 and Figure 17)

IMIN(RIPPLE) = typically 80mA

Current Rating: Finally, the inductor(s) must have a rating

greater than its peak operating current to prevent inductor

saturation resulting in efficiency loss. In steady state, the

peak input inductor current (continuous conduction mode

only) is given by:

IL1-PEAK

=

|VOUT • IOUT|

VIN • h

+

VIN • DC

2 • L1• f

for the boost, uncoupled inductor SEPIC and uncoupled

inductor inverting topologies.

For more information www.linear.com/LT8580

8580f

11

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