The high power operation of LCD and keypad backlight are not necessary in bright circumstances (for instance in daylight or a bright indoor room). The function (detection of the ambient illuminance and automatic control of the backlight) realizes low power consumption and gives longer operation time.

Advantage of  Illumination Sensor against phototransistor solution:

• Internal LEC (Luminous Efficiency Correction) Function + Amp. circuit saves mounting space on PCB
• High sensitivity - Low level light detectable
• Narrow distribution of light current - 1.67 times width max.
• Good linearity of V_out

TPS851/TPS852 Features

• Small surface-mount package
  Suitable for lead-free soldering and reflow soldering.
• High sensitivity:
  TPS851 : 52uA(typ.)
  TPS852 : 40uA(typ.)
  @EV=100lx using fluorescent light
• Narrow distribution of light current (IL):
  Light current distribution width : x1.67
  @EV=100lx using fluorescent light.
  With IL classification.
  TPS851(A) : 37uA to 62uA
  TPS851(B) : 44uA to 74uA
  TPS852(A) : 30uA to 50uA
• Built-in L.E.C. (Luminous Efficiency Correction) function:
  Sensitivity difference due to light sources is very small.
  TPS851,TPS852:IL1 /IL2 = 1.2(typ.)
  IL1 : IL(@Sensitivity for incandescent light)
  IL2 : IL(@Sensitivity for fluorescent light)
• Low supply voltage:VCC=2.7V to 5.5V