x

Where D =

(2 x L x f)

DQG '¶ = (1 - D)

'¶

For Boost

LP8862-Q1

SNVSA75A – NOVEMBER 2015 – REVISED NOVEMBER 2015

www.ti.com

Typical Applications (continued)

8.2.1.1 Design Requirements

Typical design parameters for a boost-mode two-string configuration are shown in Table 5:

Table 5. Boost Mode Design Parameters

DESIGN PARAMETER

VALUE

VIN voltage range

4.5…28 V

LED string

2 × 8 LEDs (30 V)

LED string current

160 mA

Max boost voltage

37 V

Boost switching frequency

400 kHz

External boost sync

not used

Boost spread spectrum

enabled

L1

22

μH

10 µF, 50 V

2 × (10-µF 50-V ceramic) + 33-µF 50-V electrolytic

2 × (10-µF 50-V ceramic) + 33-µF 50-V electrolytic

15 pF

1 µF, 10 V

30 k

Ω

160 k

Ω

50 m

Ω

R1

750 k

Ω

R2

130 k

Ω

R3

10 k

Ω

20 k

Ω

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor

current ripple must be small enough to achieve the desired output voltage ripple. Different saturation current

rating specifications are followed by different manufacturers so attention must be given to details. Saturation

current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of

application should be requested from the manufacturer. Shielded inductors radiate less noise and are preferred.

The saturation current must be greater than the sum of the maximum load current and the worst case average-

to-peak inductor current. Equation 4 shows the worst-case conditions:

•

•

•

•

L - min inductor value including worst case tolerances

•

f - minimum switching frequency

•

•

D - Duty Cycle for CCM Operation

•

(4)

22

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