SM8131B Datasheet, PDF(11/21 Page) Nippon Precision Circuits Inc – The SM8131B is a charge pump DC/DC converter that switches between 1 and 1.5 automatically.

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SM8131B Datasheet, PDF (11/21 Pages) Nippon Precision Circuits Inc – The SM8131B is a charge pump DC/DC converter that switches between 1 and 1.5 automatically.

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SM8131B

Switching from Ã1.0 mode to Ã1.5 mode

If the LEDs can be driven solely by the current determined by RSET and EN pulse input without boosting the

VIN input voltage, the VOUT output operates in Ã1.0 mode. In other words, if sufï¬cient current ï¬ows even with

the LED forward-direction voltage drop âVFâ, then VOUT is less than 4.2V. Operation in the more efï¬cient

Ã1.0 mode reduces the total power dissipation and extends the battery drive time. And if low âVFâ LEDs are

used or the LED drive current setting is LOW, the operating time in Ã1.0 mode is also extended.

1.0 mode

(high efficiency)

High VF LED

1.5 mode

(low efficiency)

1.0 mode

(high efficiency)

Low VF LED

1.5 mode

(low efficiency)

High

VIN input

Low

VOUT mode switching time comparison due to drive LED âVFâ variation

If the VIN voltage falls or RSET increases such that the LED drive current is too low in Ã1.0 mode, then the

LED drive circuit low-current detector operates, the VOUT output is automatically switched to Ã1.5 mode, and

the charge pump boosts the output voltage. This occurs even if only one LED low-current condition is detected

among the 6-channel LED drive circuits, thus it is recommended that LEDs have small âVFâ variation to opti-

mize the total efï¬ciency.

VIN

input voltage

Low ILED

detector

Low ILED

detected

1.0 1.5 threshold

(1)

VOUT mode

1.0 mode

VOUT

output voltage

1.5 mode

(2)

4.2V

(3)

VIN voltage drop and VOUT output voltage

(1) When VIN falls, VOUT cannot overcome the LED âVFâ rating, causing insufï¬cient current ï¬ows and the

ILED low-current signal becomes active.

(2) When the ILED low-current condition is detected, the VOUT output circuit switches to Ã1.5 mode (charge

pump mode).

(3) Prior to switching to Ã1.5 mode, the VOUT voltage may be lower than 4.2V but sufï¬cient current is sup-

plied to the LED.

If after startup, the LED connected to a DIN pin is switched, the LED connection detector circuit ï¬ags an error,

and correct mode switching may not occur. If the DIN pin does not control the LED drive current, the low-cur-

rent detector does not operate and the device cannot switch to Ã1.5 mode.

NIPPON PRECISION CIRCUITS INC.â11

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