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KTZ8866

January 2022 - Revision 04b

Page 22 of 37

Company Confidential

In case of an LED failing open, the current sink voltage of the failed string will go close to ground and dominate

the boost converter control loop. As a result, the output voltage will increase until it reaches the over-voltage

threshold set by register 0x02. Once an OVP event has been detected, the boost will stop switching and the

BL_OVP flag will be set in register 0x0F. The outcome of OVP event depends on the setting of OVP_MODE bit

in register 0x02. If OVP_MODE is set to 0, the LED open channel will not be disabled, as soon as VBL_OUT falls

below the backlight OVP threshold, the KTZ8866 begins switching again, so that VBL_OUT will be kept close to

OVP threshold. Once the opened channel resumes to connected later, its LED current will resume and VBL_OUT

will go back to normal level. If OVP_MODE is set to 1, once the over-voltage incident is triggered, the BL_OVP

flag is set in register 0x0F. Any of the enabled current sink headroom voltage drops below 150mV will be

disabled. Then the output voltage of the boost converter will go back to normal level. During the entire process,

the rest of the LED string (healthy LED string) would continue in normal operation. Even if the open channel is

reconnected later, its LED current will not resume until toggling HWEN or sending software reset command or

resetting backlight mode.

In case where all LED channels are open, once the output voltage of the boost converter reaches the over-

voltage threshold, all the current sinks will be disabled internally and the boost converter will stop switching.

User needs to restart the IC by toggling HWEN or sending software reset command or resetting backlight mode.

Backlight Over Current Protection

The KTZ8866 has 4 different OCP thresholds (1.2A, 1.6A, 2.0A, and 2.5A) chosen by register 0x11 bits [1:0]. It

is a cycle-by-cycle current limit by detecting low side power FET current. Once the threshold is trigged, the low

side power FET will be turned off immediately for the rest of the switching cycle time. If enough overcurrent

threshold events occur, the BL_OCP Flag (register 0x0F, bit [0]) will be set.

LCD Bias Boost Converter

REG pin is the output of a high efficiency boost which is used to generate OUTP and OUTN power rails. REG

boost ranges from 4V to 6.6V with 50mV step size. OUTP is generated by an LDO whose input is REG pin.

OUTP ranges from 4V to 6.3V with 50mV step size and supports up to 200mA output current. OUTN is generated

by an inverting charge pump whose input is REG pin. OUTN ranges from -6.3V to -4V with 50mV step size and

supports up to 200mA output current. Refer to 0x0C, 0x0D, 0x0E for the settings of REG, OUTP and OUTN.

For proper operation, REG voltage is suggested to be

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for lower currents and

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OUTP and OUTN voltage settings can be changed while they are enabled, but user must re-write 0x09 to get

new settings taking effect. The REG voltage changes immediately upon a register write. The LCD Bias outputs

can be turned on/off either by ENP and ENN pins or by 0x09 register bits [2:1]. EXT_EN bit in 0x09 is used to

select on/off is controlled by external pins or internal register bits. Refer to Table 4 for detailed information.

Table 4. LCD Bias Power Operating Mode

HWEN

ENN

ENP

LCD_EN_MODE

0x09[7]

OUTP_EN

0x09[2]

OUTN_EN

0x09[1]

EXT_EN

0x09[0]

ACTION

0

X

X

X

X

X

X

Device shutdown

1

0

0

0

X

X

1

Standby8

1

X

X

1

0

0

0

Standby8

1

0

1

1

X

X

1

VOUTP enabled via external input

1

1

0

1

X

X

1

VOUTN enabled via external input

1

1

1

1

X

X

1

VOUTP and VOUTN enabled via external

Input

1

X

X

1

1

0

0

VOUTP enabled via I

2C

1

X

X

1

0

1

0

VOUTN enabled via I

2C

1

X

X

1

1

1

0

VOUTP and VOUTN enabled via I

2C

8. Standby signifies that OUTP and OUTN are either high impedance or being internally pulled low via the active pulldown, and that the LCD

boost is off. Shutdown signifies that that the device was reset and no I2C communication is accepted.