LM3633 datasheet(13/58 Pages) TI2

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LM3633

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SNVS867 – JUNE 2014

Device Functional Modes (continued)

7.4.1.4 Boost Switching-Frequency Select

The LM3633 boost converter has two switching frequency settings. The switching frequency setting is controlled

via the Boost Frequency Select bit (bit 0 in the Boost Control register). Operating at the 500-kHz switching

frequency results in better efficiency under lighter load conditions due to the decreased switching losses. In this

mode the inductor must be between 10 µH and 22 µH. Operating at the 1-MHz switching frequency results in

better efficiency under higher load conditions due to in lower conduction losses in the MOSFETs and inductor. In

this mode the inductor can be between 4.7 µH and 22 µH.

7.4.1.5 Automatic Switching Frequency Shift

The LM3633 has an automatic frequency-select mode (bit 3 in the Boost Control register) to optimize the

frequency vs load dependent losses. In Auto-Frequency mode the boost converter switching frequency is

changed based on the high-voltage LED current. The threshold (Control A and Control B brightness code) at

which the frequency switchover occurs is programmable via the Auto-Frequency Threshold register. The Auto-

Frequency Threshold register contains an 8-bit code which is compared to the 8 MSBs of the brightness code.

When the brightness code is greater than the Auto-Frequency Threshold value the boost converter switching

frequency is 1 MHz. When the brightness code is less than or equal to the Auto-Frequency Threshold register

the boost converter switching frequency is 500 kHz. The default value in the Auto-Frequency Threshold register

is set for the default full-scale current setting (20.2 mA).

Figure 13 shows the LED efficiency improvement (3p5s LED configuration with a 4.7-

μH inductor) when the auto-

frequency feature is enabled. When the LED brightness is less than or equal to 0x6C, the switching frequency is

500 kHz, and it improves the LED efficiency by up to 6%. When the LED brightness is greater than 0x6C, the

switching frequency is 1 MHz, and it improves LED efficiency by up to 2.2%.

Figure 13. Auto-Frequency Boost Efficiency Improvement

Table 1 summarizes the general recommendations for auto-frequency threshold setting vs inductance values and

LED string configurations. These are general recommendations — the optimum auto-frequency threshold setting

should be evaluated for each application

Table 1. Auto-Frequency Threshold Settings

THREE STRING

TWO STRING

AUTO-

INDUCTOR

AUTO-FREQUENCY

PEAK EFFICIENCY

PEAK

PEAK EFFICIENCY

PEAK

FREQUENCY

(µH)

THRESHOLD

IMPROVEMENT (%)

CONFIGURATION

IMPROVEMENT(%)

CONFIGURATION

THRESHOLD

4.7

2.2

3p5s

1.1

2p6s

1.7

3p4s

1.3

2p5s

0.7

3p3s

0.7

2p4s

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