LT3476 datasheet(23/36 Pages) AD

23 / 36 page

LT3966

Rev. 0

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–

CDAC

S/H

VIN/48

EXT1

EXT2

FB1–4

VLED1–4

1.275V

GND

ADC

MUX

SHIFT REGISTER

CONTROL

LOGIC

AUTO

3966 F17

TGT[3:0] CLKSEL[1:0]

OUTPUT REGISTERS

0X51

VIN48

0X53

EXT1

0X54

EXT2

0X55

VFB1

0X56

ILED1

0X57

VFB2

0X58

ILED2

0X59

VFB3

0X5A

ILED3

0X5B

VFB4

0X5C

ILED4

Figure 20. Scope Shot – LATOFF

APPLICATIONS INFORMATION

tCONV

START CONVERSION

I2C STOP

SDA

CAL SAMPLE

CONVERSION N

TRANSFER

CAL SAMPLE

CONVERSION N + 1

TRANSFER

3966 F18

SCL

CHIP ADDRESS

DATA N

PEC

I2C START

I2C STOP

READ ADC RESULT

Figure 21.

When measuring a voltage, the range of the ADC is 0V to

1.275V and the LSB is 5mV.

The ADC can be used in manual (single) or automatic

mode. In manual mode, one conversion is taken for a

single target specified by the TGT[3:0] register, and the

result is stored in the target’s data register. The ADC pow-

ers down after completion. In automatic mode (RUN =

AUTO = 1), all active targets will be continuously mea-

sured in a round-robin fashion. When the last target is

completed, the converter will start again with the first

target. This ensures that fresh data is available for all

enabled targets at any given time.

Manual Mode – Repeated Conversions

In manual mode (RUN = 1, AUTO = 0), multiple repeated

conversions on the same target can be performed with lit-

tle overhead. The first conversion starts upon detection of

the I2C Stop signal in the transfer in which the ADC’s RUN

bit is written to 1. The ADC completes one conversion

and enters sleep mode. Reading the ADC result without

clearing the RUN bit will trigger a new conversion at the

I2C Stop signal following the data read. In this manner,

an unlimited number of conversions and reads can be

executed in series.

When repeated conversions are used, a conversion time

(tCONV) of at least 20ADC clock periods between an I2C

Stop and the next I2C Start is required to complete the

conversion before data can be read. This time is depen-

dent upon RT and the value in the ADC’s CLKSEL register

bits. If a new I2C Start occurs before the data is ready,

the data read will be the prior conversion’s data, and the

new data will be transferred upon the next I2C Stop. The

appropriate timing is shown in Figure 21.

Automatic Mode

The ADC will operate in automatic mode when the AUTO

bit is set. In this mode, the ADC runs continuously, mea-

suring each active input and updating the target data reg-

ister. This sequence repeats indefinitely, and does not wait

for an I2C Stop to begin the next round of conversions.

Additionally, data can be read at any time. In this man-

ner, automatic mode will always provide fresh data for all

active targets.

In the case of a disabled channel (OFF = 1), both the FB

and ILED measurements for that channel will be skipped.

For a channel that is enabled, but is in a PWM off time

from either CTRL/PWM or dimming generator low time,

the converter will not skip that channel but will instead wait

for the channel’s PWM signal to rise before sampling. Be

aware that when using CTRL/PWM for external dimming,

long off times in the external signal will result in equivalent

pauses in the ADC converter’s measurement sequence.

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