ADM8845ACPZ-REEL7;ADM8845ACPZ-REEL;ADM8845EB-EVALZ;中文规格(2)

35

30LED CURRENT (mA)

25

MATCHING ERROR (%)

04867-0-007

20

15

10

04867-0-004

54.75

6.75

10.758.75

RSET (k )

12.7514.75

2.6

3.03.4

3.84.24.6SUPPLY VOLTAGE (V)

5.05.4

Figure 3. ILED (mA) Current vs. RSET

20.3520.30

LED CURRENT (mA)

Figure 6. ILED (mA) Matching Error (%) vs. Supply Voltage (V),

TA = 25°C and ILED = 20 mA

LED CURRENT (mA)

20.2520.20

20.15

20.1020.0520.00

2.6

04867-0-005

–40

04867-0-00804867-0-009

3.1

4.13.64.6

SUPPLY VOLTAGE (V)

5.1

5.6

40

TEMPERATURE ( C)

80

Figure 4. ILED (mA) vs. Temperature (°C), Six LEDs Enabled

0.3

Figure 7. ILED (mA) Variation over Temperature (°C), VCC = 3.6 V

35

0.2

30

LED CURRENT (mA)

04867-0-006

0.1

25

% ERROR

20

–0.1

15

–0.2

10

–0.3

–40–20

04525

TEMPERATURE ( C)

6585

52.6

3.03.4

3.84.24.6SUPPLY VOLTAGE (V)

5.05.4

Figure 5. ILED Matching (%) over Temperature (°C), VCC = 3.6 V, ILED = 20 mA,

Six LEDs Enabled

Figure 8. ILED (mA) vs. Supply Voltage (V)

Rev. C | Page 6 of 20

95

20

90

16

LED CURRENT (mA)

85

EFFICIENCY (%)

04867-0-010

12807570

8

4

6560

04867-0-013

020

4060DUTY CYCLE (%)

80100

0102030

405060DUTY CYCLE (%)

708090100

Figure 9. ILED (mA) vs. PWM Dimming (Varying Duty Cycle),

Six LEDs Enabled, Frequency = 1 kHz

300

Figure 12. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal,

Six LEDs Enabled, 20 mA/LED

250

SUPPLY CURRENT

ICC (mA

)

200

150

100

02.6

04867-0-011

3.03.4

3.84.24.6SUPPLY VOLTAGE (V)

5.05.4

CH3 1.00V

04867-0-014

50

V

Figure 10. Input Current vs. Supply Voltage,

Six LEDs Enabled Figure 13. Soft Start Showing the Initial In-Rush Current and VOUT Variation,

Six LEDs @ 20 mA/LED, VCC

= 3.6 V

VCCVCC

VOUT

04867-0-012

VOUT

04867-0-015

W

W

M 400ns

W

W

M 400ns

Figure 11. 1.5× Mode Operating Waveforms Figure 14 .2× Mode Operating Waveform

Rev. C | Page 7 of 20

ADM8845

908580POWER EFFICIENCY (%)

757065605550

VCC

VOUT

04867-0-016

45

40

VCC(V)

04867-0-01904867-0-020

W

CH2

W

M 400ns

Figure 15. 1× Mode Operating Waveforms

908580POWER EFFICIENCY (%)

Figure 18. Power Efficiency vs. Supply Voltage over Li-Ion Range,

Six LEDS @ 20 mA/LED

90

POWER EFFICIENCY (%)

858075706560555045

04867-0-017

75706560555045

40

VCC(V)

40

VCC(V)

Figure 16. Power Efficiency vs. Supply Voltage over Li-Ion Range,

Six LEDS @ 15 mA/LED

908580POWER EFFICIENCY (%)

Figure 19. Power Efficiency vs. Supply Voltage over Li-Ion Range

Four LEDS @ 20 mA/LED

75706560555045

V40

VCC(V)

04867-0-018

2.00VCH2 2.00VM10.0ms04867-0

-021

Figure 20. TPC Delay

Figure 17. Power Efficiency vs. Supply Voltage over Li-Ion Range,

Four LEDS @ 15 mA/LED

Rev. C | Page 8 of 20

30mA

20mA

65°C85°C

04867-0-022

Figure 21. Maximum ILED (mA) vs. Ambient Temperature,

Six LEDs Connected

Rev. C | Page 9 of 20

An external resistor, RSET, is connected between the ISET pin and GND. This resistor sets up a reference current, ISET

, which is internally gained up by 120 within the ADM8845 to produce the ILED currents of up to 30 mA/LED (ILED = ISET × 120 and ISET = 1.18 V/RSET). The ADM8845 uses six individual current sinks to individually sense each LED current with a maximum matching performance of 1%. This current matching perform-ance ensures uniform brightness across a color display. The ADM8845 lets the user control the brightness of the white LEDs with a digital PWM signal applied to CTRL1 and/or CTRL2. The duty cycle of the applied PWM signal determines the brightness of the main and/or sub display backlight white LEDs. The ADM8845 also allows the brightness of the white LEDs to be controlled using a dc voltage (see Figure 22). Soft-start circuitry limits the in-rush current flow at power-up. The ADM8845 is fabricated using CMOS technology for minimal power consumption and is packaged in a 16-lead lead frame chip scale package.

THEORY OF OPERATION

The ADM8845 charge pump driver for LCD white LED back-lights implements a multiple gain charge pump (1×, 1.5×, 2×) to maintain the correct voltage on the anodes of the LEDs over a 2.6 V to 5.5 V (Li-Ion) input supply voltage. The charge pump automatically switches between 1×/1.5×/2× modes, based on the input voltage, to maintain sufficient drive for the LED anodes, with VCC input voltages as low as 2.6 V. It also includes regula-tion of the charge pump output voltage for supply voltages up to 5.5 V. The six LEDs of the ADM8845 are arranged into two groups, main and sub. The main display can have up to four LEDs, FB1 to FB4, and the sub display can have one or two LEDs, FB5 and FB6 (see Figure 23). Two digital input control pins, CTRL1 and CTRL2, control the shutdown operation and the brightness of the main and sub displays (see Table 4).

V04867-0-027

BRIGHT

C11µF

C24.7µR

Rev. C | Page 10 of 20

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