LT8500ETJ#TRPBF_技术文档

LT8500

48-Channel LED PWM Generator

with 12-Bit Resolution and 50MHz

Cascadable Serial Interface

FeaTures

DescripTion

The LT^®8500 is a pulse width modulation (PWM) genera-

tor with 48 independent channels. Each channel has an

individually adjustable 12-bit (4096-step) PWM register

anda6-bit(64-step) 50ꢀcorrectionregister.Allcontrols

are programmable via a simple serial data interface. Three

banks of 16-channels each can be configured such that

n

3V to 5.5V Input Voltage

n

48 Independent PWM Outputs

n

TTL/CMOS Logic 50MHz Serial Data Interface

n

12-Bit (4096 Steps) PWM Width Resolution

6-Bit (64 Steps) PWM Correction

( 50ꢀ of Prograꢁꢁed PWM Width)

Up to 6.1kHz PWM Frequency (PWMCK = 25MHz)

Phase-Shift Option Reduces Switching Noise

Directly Controls Three LT3595A 16-Channel

LED Drivers

Diagnostic Information: Sync Error/Open LED Flags

56-Pin (6mm × 6mm × 0.78mm) TLA QFN Package

n

they operate 120° out-of-phase with each other.

n

The LT8500 features two diagnostic information flags:

synchronization error and open LED. The flags are sent,

withadditionalstateinformation,ontheserialdatainterface

duringstatusreadback.The50MHzcascadableserialdata

interface includes buffering and skew-balancing, making

the chip suitable for PWM intensive applications such

as large screen LCD dynamic backlighting and mono-,

multi- and full-color LED displays. The LT8500 is also

ideally suited to control three LT3595A LED drivers.

n

applicaTions

n

Large Screen Display LED Backlighting

n

Mono-, Multi-, Full-Color LED Displays

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

n

LED Billboards and Signboards

Motor Control

Industrial Control

Automated Test Equipment

Robotics

n

Typical applicaTion

48 PWM OUTPUTS

• • • • •

PWM[48:1]

5-WIRE

SERIAL

• • • •

SDI

SCKI

SDO

SCK0

SDI

SCKI

SDO

SCK0

SDI

SCKI

SDO

SCK0

DATA

INTERFACE

LDIBLANK

LT8500 (1)

LDIBLANK

LT8500 (2)

LT8500 (N)

OSC

PWMCK

OPENLED

DIAGNOSTIC

CIRCUIT

• • • •

8500 TA01a

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LT8500

absoluTe MaxiMuM raTings ^{(Note 1)}

V ............................................................... –0.3V to 6V

Operating Junction Temperature Range

CC

SDI, SCKI, PWMCK, OPENLED,

(Note 2).................................................. –40°C to 125°C

Storage Temperature Range .................. –65°C to 150°C

LDIBLANK......... –0.3V to Lesser of 6V and (V + 0.3V)

CC

pin conFiguraTion

TOP VIEW

56 55 54 53 52 51 50 49 48 47

PWM1

PWM24

PWM23

PWM22

PWM21

PWM28

PWM27

PWM26

PWM25

1

2

3

4

5

6

7

8

9

46 PWM12

45 PWM5

PWM6

PWM7

44

43

PWM2

PWM24

PWM22

PWM23

PWM26

PWM25

A1 A40 A39 A38 A37 A36 A35 A34 A33 A32 A31

PWM12

PWM7

42 PWM8

A2

A3

A4

A5

A6

A7

A8

A9

A10

A30

A29

A28

A27

A26

A25

A24

A23

A22

PWM1

PWM21

PWM27

PWM28

PWM32

PWM30

PWM19

PWM18

B1

B2

B3

B4

B5

B6

B7

B8

B16

B15

B14

B13

B12

B11

B10

B9

PWM5

PWM8

SDI

41

LDIBLANK

PWM6

40 SDI

LDIBLANK

PWMCK

39 PWMCK

38 SCKO

37 VCC

57

GND

SDKO

57

GND

PWM32 10

PWM31 11

PWM30 12

PWM29 13

PWM20 14

PWM19 15

PWM18 16

PWM17 17

PWM40 18

V

CC

SCKI

36 SCKI

PWM31

PWM20

PWM29

PWM17

PWM40

SDO

OPENLED

PWM35

PWM45

35 SDO

PWM33

PWM34

PWM36

PWM46

34 OPENLED

33 PWM33

32 PWM34

31 PWM35

30 PWM36

29 PWM45

A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21

19 20 21 22 23 24 25 26 27 28

TJ PACKAGE

56-LEAD PLASTIC TLA QFN (6mm × 6mm)

T

= 125°C, θ = 35°C/W, θ = 5°C/W

JA JC

JMAX

UHH PACKAGE

56-LEAD (5mm × 9mm) PLASTIC QFN

EXPOSED PAD (PIN 57) IS GND, MUST BE SOLDERED TO PCB

T

= 125°C, θ = 35°C/W, θ = 5°C/W

JA JC

JMAX

EXPOSED PAD (PIN 57) IS GND, MUST BE SOLDERED TO PCB

(NOT RECOMMENDED FOR NEW DESIGNS)

http://www.linear.coꢁ/product/LT8500#orderinfo

orDer inForMaTion

LEAD FREE FINISH

LT8500EUHH#PBF

LT8500IUHH#PBF

LT8500ETJ#PBF

LT8500ITJ#PBF

TAPE AND REEL

PART MARKING*

8500

PACKAGE DESCRIPTION

TEMPERATURE RANGE

–40°C to 125°C

LT8500EUHH#TRPBF

LT8500IUHH#TRPBF

LT8500ETJ#TRPBF

LT8500ITJ#TRPBF

56-Lead (5mm × 9mm) Plastic QFN

56-Lead (6mm × 6mm) Plastic TLAQFN

8500

LT8500TJ

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through

designated sales channels with #TRMPBF suffix.

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LT8500

elecTrical characTerisTics ^Thel denotes the specifications which apply over the full operating

teꢁperature range, otherwise specifications are at T_A= 25°C. V_CC= 3.3V, unless otherwise noted.

SYMBOL

Supply

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNIT

l

V

CC

V

CC

Operating Voltage

3.0

5.5

V

Digital Inputs: SCKI, SDI, LDIBLANK, OPENLED, PWMCK

Input Logic Levels

l

V

High Level Voltage

V

= 5V

4.0

2.7

V

IH

CC

= 3.3V

= 5V

Low Level Voltage

1.0

0.6

IL

= 3.3V

I

Input Current

Pin Voltage = V or GND Excluding OPENLED

–1

70

1

µA

kΩ

pF

IN

CC

R

OPENLED Pull-Up Resistor

V

= 5.5V

CC

100

3

130

PU

IN

C

Input Capacitance (Note 4)

Pin to GND

Digital Outputs: SCKO, SDO, PWM[48:1]

SDO, SCKO Output Voltages

l

V

High Level Voltage

I

= –6mA, V = 5V

4.0

2.7

V

OH

OUT

CC

= –3mA, V = 3.3V

CC

Low Level Voltage

= 6mA, V = 5V

1.0

0.6

OL

CC

= 3mA, V = 3.3V

CC

PWM [48:1] Output Voltages

High Level Voltage

l

V

I

= –3mA, V = 5V

4.0

2.7

V

OH

OUT

CC

= –1.5mA, V = 3.3V

CC

Low Level Voltage

= 3mA, V = 5V

1.0

0.6

OL

CC

= 1.5mA, V = 3.3V

CC

TiMing characTerisTics ^Thel denotes the specifications which apply over the full operating teꢁperature

range, otherwise specifications are at T_A= 25°C. V_CC= 3.3V, and all inputs are rail-to-rail unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

50

UNIT

MHz

ns

l

f

Data Shift Clock Frequency

PWMCK Clock Frequency

Minimum SCKI High Time (Note 3)

Minimum SCKI Low Time (Note 3)

SCKI ↑ – SCKI ↑ (Figure 4)

PWMCK ↑ – PWMCK ↑ (Figure 5)

SCKI = High

SCKI

f

25

PWMCK

t

2

WH-SCKI

WL-SCKI

WH-LDI

SCKI = Low

ns

l

LDIBLANK Pulse Duration

(LDI Function)

LDIBLANK = High (Figure 4)

8

5,000

ns

t

LDIBLANK Pulse Duration

(BLANK Function)

LDIBLANK = High (Figure 4)

50,000

ns

WH-BLANK

l

t

SDI-SCKI Setup Time (Note 3)

SCKI-SDI Hold Time (Note 3)

3

ns

SDI ↑↓ – SCKI ↑ (Figure 4)

SCKI ↑ – SDI ↑↓ (Figure 4)

SU-SDI

HD-SDI

SU-LDI

1.75

10

SCKI-LDIBLANK Setup Time (Note 3)

SCKI ↑ – LDIBLANK ↑ (Figure 4)

SCKI 50ꢀ Duty Cycle

l

t

LDIBLANK-SCKI Hold Time (Note 3)

SCKI-SDO Propagation Delay (Note 3)

SCKI-SCKO Propagation Delay (Note 3)

SCKO-SDO Hold Time (Note 3)

5

ns

LDIBLANK ↓ – SCKI ↑ (Figure 4)

SCKI ↑ – SDO ↑↓ (Figure 4)

HD-LDI

PD-SDO

PD-SCK

HD-SDO

DC-SCK

15

10

25

20

SCKI ↑ – SCKO ↑ (Figure 4)

2.75

SCKO ↑ – SDO ↑↓ (Figure 4)

Difference Between SCKI = High Time

SCKI-SCKO Duty Cycle Change (Note 4)

–0.2

and SCKO = High Time, C

= 25pF

LOAD

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LT8500

TiMing characTerisTics ^Thel denotes the specifications which apply over the full operating teꢁperature

range, otherwise specifications are at T_A= 25°C. V_CC= 3.3V, and all inputs are rail-to-rail unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNIT

l

t

PWMCK-PWM[48:1] Propagation Delay

(Note 3)

32

50

ns

PWMCK ↑ – PWM ↑↓ (Figure 5)

PD-PWM

t

SDO, SCKO Rise Time (Note 4)

SDO, SCKO Fall Time (Note 4)

PWM[48:1] Rise Time (Note 4)

PWM[48:1] Fall Time (Note 4)

C

LOAD

C

LOAD

C

LOAD

C

LOAD

= 25pF, 30ꢀ to 70ꢀ

= 25pF, 70ꢀ to 30ꢀ

= 25pF, 30ꢀ to 70ꢀ

= 25pF, 70ꢀ to 30ꢀ

2

ns

R-SDO

F-SDO

12

R-PWM

F-PWM

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

characterization and correlation with statistical process controls. The

LT8500I is guaranteed over the full –40°C to 125°C operating junction

temperature range.

Note 3: Propagation delays, setup/hold times and hi times are measured

Note 2: The LT8500E is guaranteed to meet performance specifications

from 0°C to 125°C junction temperature. Specifications over the –40°C

to 125°C operating junction temperature range are assured by design,

from 50ꢀ to 50ꢀ.

Note 4: This parameter is correlated to lab measurements and is not

subject to production testing.

TiMing DiagraM

t

WH-SCKI

WH-LDI

t

1/f

SCKI

HD-SDI

t

SU-LDI

HD-LDI

WL-SCKI

t

SU-SDI

SCKI

SDI

LDIBLANK

SCKO

SDO

8500 TD01

t

PD-SDO

PD-SCK

HD-SDO

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LT8500

Typical perForMance characTerisTics

For the I_CCvs V_CCGraphs, the Following Conditions

Apply: 23pF Load on SCKO. PWM Outputs Enabled: Duty Cycle = 1365/4096, 10pF Average Load on PWMs.

I_CCvs V_CC, SCKI = 0MHz,

SDI = 0MHz, PWMCK = 0MHz

I_CCvs V_CC, SCKI = 0MHz,

SDI = 0MHz, PWMCK = 10MHz

I_CCvs V_CC, SCKI = 0MHz,

SDI = 0MHz, PWMCK = 25MHz

3.0

2.5

2.0

1.5

1.0

0.5

0

3.0

2.5

2.0

1.5

1.0

0.5

0

3.0

2.5

2.0

1.5

1.0

0.5

0

3

3.5

4

4.5

(V)

5

5.5

3

3.5

4

4.5

(V)

5

5.5

3

3.5

4

4.5

(V)

5

5.5

V

CC

V

CC

V

CC

8500 G01

8500 G02

8500 G03

I_CCvs V_CC, SCKI = 12MHz,

SDI = 6MHz, PWMCK = 0MHz

I

_CCvs V_CC, SCKI = 20MHz,

I_CCvs V_CC, SCKI = 50MHz,

SDI = 25MHz, PWMCK = 25MHz

SDI = 10MHz, PWMCK = 10MHz

30

25

20

15

10

5

30

25

20

15

10

5

30

25

20

15

10

5

0

3

3.5

4

4.5

(V)

5

5.5

3

3.5

4

4.5

(V)

5

5.5

3

3.5

4

4.5

(V)

5

5.5

V

CC

V

CC

V

CC

8500 G06

8500 G04

8500 G05

V_OLvs V_CC

V_OHvs V_CC

0.6

0.5

0.4

0.3

0.2

0.1

0

0.6

0.5

0.4

0.3

0.2

0.1

0

PWMs AT 3mA

SCKO, SDO AT 6mA

PWMs AT 1.8mA

PWMs AT 3mA

SCKO, SDO AT 6mA

PWMs AT 1.8mA

0

1

2

3

4

5

6

0

1

2

3

4

5

6

V

CC

(V)

V

(V)

CC

8500 G07

8500 G08

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LT8500

pin FuncTions ^(TLAQFN/QFN)

PWM[48:1] (Pins A1 to A24, A29 to A40, B1 to B10, B15

to B16/Pins 1 to 33, 42 to 56): Pulse Width Modulated

(PWM) Output Pins. Pulse width is determined by com-

paring the value in the PWMRSYNC latches to an internal

PWMCK counter. Outputs are high when the value in the

PWMCK counter is less than the value in PWMRSYNC[n].

The PWM frequency is determined by the signal applied

to the PWMCK pin.

PWMCK (Pin A27/Pin 39): PWM Clock Input Pin. This pin

provides PWM timing for the outputs. Each PWM signal

is generated by counting the pulses on this clock from

zero to the calculated value in the PWM synchronization

register(PWMRSYNC). ThisclockisindependentofSCKI.

SDI (Pin B14/Pin 40): Serial Data Input Pin. This pin

provides serial interface data to issue commands and set

up the individual PWM channels.

OPENLED (Pin B11/Pin 34): Not Open LED Input Pin.

This input passes diagnostic information to the host via

the status frame. When used with LT3595A LED drivers,

it connects to the wired-OR (open collector) OPENLED

outputs which indicate an open in one or more of the

LED strings. The user can run a self test on the LT8500

to detect which PWM output is associated with an open

LED string, or other fault. This pin has an internal 100kΩ

LDIBLANK (Pin A28/Pin 41): Latch Data In/Blank Input

Pin. This is a dual function pin.

LDI Function: The internal LDI signal is directly con-

nected to the LDIBLANK pin. A logic high on the pin

always asserts the LDI function. The rising edge of

LDIBLANKcapturesthedecodedcommandfield(CMD,

CR[7:0]) of the shift register (SR[7:0]). The high level

ofLDIBLANKlatchesdatafromthecorrectionmultiplier

into the PWM Registers (PWMR). When LDIBLANK is

high, status information is loaded into the shift register

(SR) to shift out on SDO when the next frame shifts in

on SDI. See more details in the Operation section.

pull-up to the V supply rail.

CC

SDO (Pin A25/Pin 35): Serial Data Output Pin. This pin is

the output of the shift register (SR), and cascades data to

downstream chips or returns data to the host.

SCKI (Pin B12/Pin 36): Serial Clock Input Pin. This clock

pin provides timing for the serial interface and the calcula-

tion of PWM values in the correction multiplier. This clock

is independent of PWMCK.

BLANK Function: Asserting LDIBLANK high for more

than 50µs turns off all PWM[48:1] outputs and resets

the chip. To avoid inadvertently resetting the chip, do

not assert LDIBLANK high for more than 5µs.

V

(Pin A26/Pin 37): Supply Pin. 3.0V to 5.5V. Must be

CC

GND (Exposed Pad Pin 57): This is the ground reference

for the chip.

locally bypassed with a capacitor to ground.

SCKO (Pin B13/Pin 38): Serial Clock Output Pin. Buffered

pass through of SCKI. This pin cascades the clock to the

next chip or to the host.

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LT8500

block DiagraM

V

CC

37

34

100k

OPENLED

CRD, PHS, SYC,

OLT, CR[4:7]*

STATUS (COR’s, OLED’s)

PD

SDI

40

36

SDO

SD

35

LD

SHIFT REGISTER (SR[0:583])*

POR

SCKI

CR[0:7]* FRAME DATA (SR[8:583])*

8

288 = {SR[14:19], SR[26, 31],..., SR[578:583]}*

6

LD

LDIBLANK

PWMCK

COR[n]

41

39

CTRL

×48

6

288

CORRECTION

MULTIPLIER

SEL

LD

576

SCKO

38

PWM CHANNEL

12

PWMR[n]

12

EN

PWMxx

×48

12

PWMRSYNC[n]

12-BIT PWM

GENERATION

GND

57

PWMCK

COUNTER

8500 BD

*REVERSE INDEXING IS USED TO INDICATE PHYSICAL BIT ORDER.

Figure 1. Block Diagraꢁ

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LT8500

operaTion

OVERVIEW

Update frames are used to serially load the 12-bit values

for each of the 48 PWM channels. The LT8500 contains

a correction multiplier that can automatically scale the

12-bit PWM channel data before it’s stored. By default,

the correction multiplier is enabled and scales incoming

channel data according to:

The LT8500 controls 48 pulse width modulated

(PWM[48:1]) outputs, suitable for control applications

such as driving three LT3595A LED drivers. The chip’s

operation is best understood by referring to the Block

Diagram in Figure 1.





 

 

3

COR_n+ 32

64

2

The major blocks inside the LT8500 are: a 584-bit

shift register (SR[0:583]), 48 6-bit correction registers

(COR[1:48]), a correction multiplier, 48 PWM channels

and a PWMCK clock counter. Each PWM channel stores

data for the associated PWMx output pin and includes a

PWM register (PWMR) and a PWM synchronization reg-

ister (PWMRSYNC). The lower 8 bits of the 584-bit shift

register are the command register (CR[0:7]) and the rest

of the shift register contains the frame data.

PWM_OUTn= CHAN_n(NOM)

•

 









where PWM

is the number of PWMCK cycles that

OUTn

PWMn is high, CHAN

is the nth channel field in

n(NOM)

the frame, and CORn is the nth programmed correction

setting (CORn = 0 to 63). See Table 1 for examples.

Otherwise, when the correction multiplier is disabled, the

incoming data is stored unchanged:

PWM

= CHAN

n(NOM)

A comparison of a channel’s PWMRSYNC register to the

PWMCK counter generates the respective PWM output

signal. The input of the 584-bit shift register (SR[0]) is

connected to the SDI signal. SDI is also an input to the

correctionmultiplier.Theoutputofthe584-bitshiftregister

(SR[583]) is connected to SDO.

OUTn

The correction multiplier is disabled by the correction

register disable bit (CRD), which is toggled by the cor-

rection toggle command (CMD = 0x7X). By default, the

correction multiplier is enabled after power-up and the

CRD bit is low.

The user communicates with the part by controlling the

serialinterfacepinsSDI,SCKIandLDIBLANK.Aserialdata

frame, called a command frame, is shifted into the part

on SDI using SCKI as the clock signal. At the same time,

the status frame is shifted out on SDO. A rising edge on

the LDIBLANK pin terminates a frame. A frame consists

of a 12-bit data field for each PWM channel, followed

by an 8-bit command field, totaling (12 × 48) + 8 = 584

bits. The data is transꢁitted with the ꢁost significant

channel first, and each field is transꢁitted MSB first.

The frame formats and timing are illustrated in Figures 3

and 4, respectively. There are eight commands, two of

whichupdatethePWM[48:1]outputs. Thecommandsare

summarized in Table 2. Within this document, command

frames will be referred to by the commands they issue,

such as “update frame” or “correction frame.”

The result generated by the correction multiplier moves

to the respective PWMRSYNC register after an update

frame. An update frame does this either synchronously or

asynchronously. A synchronous update frame will copy

the data to the PWMR on the subsequent rising edge of

LDI which marks the end of the frame, and then from the

PWMR to the PWMRSYNC register at the beginning of a

PWM period. A PWM period starts when the free-running

PWMCK counter is zero. Otherwise, the asynchronous

update frame will copy the data from the correction mul-

tiplier, through the PWMR to the PWMRSYNC at the same

time, on the subsequent rising edge of LDI which marks

the end of the frame.

As soon as the PWMRSYNC registers are updated with

their new values, the PWM outputs will reflect the update.

As mentioned earlier, the PWMR outputs are generated

by comparing the respective PWMRSYNC values to the

PWMCK counter.

With a 50MHz SCKI, a single frame can be transmitted in

11.7µs (584 SCKIs + LDI), for a frame rate of 85.5kHz.

A 25MHz PWMCK creates a PWM period (4096 PWMCKs)

of 164µs, or a PWM output frequency of 6.1kHz.

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LT8500

operaTion

START-UP

SERIAL DATA INTERFACE

The LT8500 is ready to communicate after power-up, if

the LDIBLANK pin is low. The PWM[48:1] outputs remain

disabled (logic 0) until an output enable frame is sent. The

recommended sequence of events for start-up is:

The LT8500 has a 50MHz cascadable serial data interface

with full buffering and skew balancing on clock and data.

The interface uses a novel 5-wire (LDIBLANK, SCKI, SDI,

SCKO, and SDO) topology and can be connected to a

variety of digital controllers, such as microcontrollers,

digital signal processors (DSPs), or field programmable

gate arrays (FPGAs).

1. Apply power and drive LDIBLANK low. SDO will go low

when the on-chip power-on-reset (POR) de-asserts.

2. Send a correction register frame (CMD = 0x20) on the

serial interface. This sets the correction factor on each

channel.

Topology

TwotopologiesshowninFigure2aresupportedforcascad-

ing the LT8500. For higher speeds and a large number of

LT8500s, consider the novel 5-wire topology. For lower

speedsandfewLT8500s,considertheconventional4-wire

topology. Whichever topology is used, signal integrity

should be carefully evaluated, especially for the clocks.

3. Send an update frame (CMD = 0x00 or CMD = 0x10)

on the serial interface. This sets the pulse width of each

channel.

4. Send an output enable frame (CMD = 0x30) on the

serial interface. This enables the modulated pulses on

the PWM[48:1] outputs.

The 5-wire topology eliminates the need for global SCKI

routing and reduces the need for buffer insertion for the

SCKI signal. Instead, it provides the SCKO signal along

with the SDO signal to drive the next chip. The skew inside

the chip between the SCKI and SDI signals is balanced

internally. The skew outside the chip between the SCKO

and SDO signals can be easily balanced by parallel routing

The PWM clock (PWMCK) should be turned on before

step 4. The start of a PWM period, when all PWM[48:1]

channels turn on, is synchronized to the output enable

frame when the outputs are disabled prior to the frame.

Novel 5-Wire Topology

LDI

SCKI

SDI

LDI

SCKI

SDI

LDI

SCKI

SDI

LDI

SCKI

SDI

SCK0

SDO

LT8500 (1)

SCK0

SDO

LT8500 (2)

SCK0

SDO

LT8500 (N)

HOST

CONTROLLER

SDO

SCKO

Conventional 4-Wire Topology

LDI

SCKI

SDI

LDI

SCKI

SDI

LDI

SCKI

SDI

LDI

SCKI

SDI

SDO

LT8500 (1)

LT8500 (2)

LT8500 (N)

HOST

CONTROLLER

SDO

8500 F02

Figure 2. Serial Interface Topologies

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these two signals between chips. When properly balanced

in this way, the SCKO/SDO timing will meet the timing

requirements of SCKI/SDI on the next cascaded chip,

enabling faster clock speeds and more chips in cascade.

The host controller sends the SDI signal with the SCKI

signal, and receives the SDO signal with the SCKO signal.

The controller will see skew between SCKI and SCKO,

and will need to operate on two clock planes depending

on the number of cascaded LT8500s and system timing

significant channel first, and each field is transmitted with

MSB first. The command frames are sent with the SCKI

signal and the status frame is received with the SCKO

signal. The command field determines the function of a

frame, according to Table 2. The status frame consists of

the four MSB’s of the last command (CR[7:4]), the open

LED self test bit (OLT), the synchronization error status

bit (SYC), the phase-shift status bit (PHS), the correction

register disable status bit (CRD), and individual OPENLED

fault bits (NOL[48:1]), as well as each 6-bit correction

register (COR[48:1]). Logic zeros fill in the unused bits

of the status frame. Refer to Figure 3.

constraints. A duty cycle change (t

) will also occur

DC-SCK

between SCKI and SCKO, limiting the number of LT8500s

in a chain, depending on SCKI speed. This change results

from a slight difference in propagation delays of the posi-

tive and negative edges of SCKI. LDIBLANK skew between

chips may require balancing in timing critical systems,

otherwise the host should increase the delay between

SCKI and LDI to avoid violating LDI to SCKI setup and

Figure 4 illustrates the timing relationship among serial

input and serial output signals in more detail. One correc-

tion register frame followed by an update frame is sent

through the SDI, SCKI, and LDIBLANK pins. At the same

time, two status frames are received through the SDO,

SCKO, and LDIBLANK pins. The rising edges of SCKI shift

a frame of data into shift register SR[0:583]. After 584

clock cycles, all bits of data sit in the shift register waiting

for the LDI signal. An asynchronous LDIBLANK “high”

signal captures the decoded 8-bit CMD field (CR[7:0]),

executing commands and routing data accordingly. At

the same time, a frame of status information, including

the 4 MSB’s of the CMD field (CR[7:4]), status bits, COR

registers, and individual open LED fault flags, is parallel

loaded into the 584-bit shift register and will be shifted

out as the next frame shifts in.

hold times (t

and t

). In summary, the 5-wire

HD-LDI

SU-LDI

topology extends the maximum number of cascadable

chips, boosts the series data interface clock frequency,

eliminates global SCKI routing, reduces the need of buf-

fer insertion for SCKI signals, and offers an easier PCB

layout. In a low-speed application with a small number of

cascaded chips, the 5-wire topology can be simplified to

the 4-wire topology by ignoring the SCKO output.

In a 4-wire topology, the LDIBLANK and SCKI signals

need global routing while the SDI signal only needs local

routing between chips. SCKO is ignored. When a large

number of chips are in cascade, or long board traces

are used, external clock-tree buffers with corresponding

driving capability might be needed for the LDIBLANK and

SCKI signals to minimize signal skews. The propagation

delay caused by the buffer insertion on the SCKI signal

yields the skew between the SCKI and SDI signals, which

usually requires balancing. Since both the SDI and SDO

signals require the same SCKI signal to send and receive,

the propagation delay between the SDI and SDO signals

limits the number of chips in cascade and the series data

interface clock frequency.

LDIBLANK = LDI + BLANK

The LDIBLANK pin is a dual function input, determined by

the duration of a logic high on the pin. LDI is the latch data

input, which signals the end of a frame and executes the

command in the CMD field (CR[7:0]). The BLANK signal

turns off the PWM[48:1] outputs and performs a global

reset of the part, including the shift register in the serial

interface. A logic high on LDIBLANK always asserts LDI,

while a logic high greater than the minimum LDIBLANK

pulsedurationforBLANK(t

)alsoassertsBLANK.

WH-BLANK

BLANKwillneverbeassertedifthepinisheldhighlessthan

Coꢁꢁunication

the maximum LDIBLANK pulse duration for LDI (t

).

WH-LDI

Figure 3 shows two command frames sent on SDI, and

one status frame received on SDO. All the frames have

the same 584-bit length and are transmitted with the most

Between maximum t

and minimum t

,

WH-LDI

WH-BLANK

BLANK becomes asserted at an undetermined time.

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A rising edge on the LDI signal is always interpreted as

the end of a frame. The next rising edge of SCKI after the

falling edge of LDIBLANK is always interpreted as the start

of a new frame. An out-of-sync error bit (SYC) is provided

in the status frame to alert the system if the part saw an

LDI unexpectedly. This occurs when LDI and SCKI are

both hi, or when LDI is hi on other than a frame boundary

(n • 584 SCKI’s). The SYC bit is for information only, it has

no other effect on the part. If the SYC bit is set, none of

the other data in the status frame is reliable and the effect

of the prior frame is unknown; the LT8500 assumes the

system’s timing of the LDI is correct and considers the

next SCKI as the start of the next frame.

counter is free-running from the PWMCK clock when

outputs are enabled. When an output enable frame is sent,

the PWMCK counter increments to one on the second ris-

ing edge of PWMCK after the rising edge of LDIBLANK, as

shown in Figure 5. By default, all outputs with non-zero

values in PWMRSYNC will turn on when the PWMCK

counter is one. Alternatively, if the phase-shift bit (PHS)

is set, the PWM[48:1] outputs will turn on as illustrated in

the phase-shift synchronous updates in Figure 6, case A.

Further discussion of the phase-shift function follows.

Each subsequent rising edge of PWMCK increases the

PWMCK counter by one. Any PWM channel will be turned

off when its PWMRSYNC value is equal to the value in

the PWMCK counter. An output disable frame resets the

PWMCK counter immediately after LDI, and turns off all

thePWMchannelsonthenextrisingedgeofPWMCKafter

LDI. Figure 5 shows the PWM output enable timing chart.

OPENLED

The OPENLED pin provides status information to the

host by reporting its state in the status frame. The state

of the pin is captured by each rising edge of PWMCK and

is reported in two ways. In typical use, the status frame

receives the captured state of the pin on the rising edge

of the first SCKI after LDIBLANK goes low. This state is

duplicated 48 times and reported in the LSB of each PWM

channel in the status frame. The state will normally be a

logic “1” due to the on-chip pull-up resistor.

1

t

PD-PWM

f

PWMCK

0

1

2

3

PWMCK

LDI, CMD = 0x30

8500 F05

PWM

Alternatively, the LT8500 supports a diagnostic self test

frame (CMD = 0x5X) that reports the OPENLED state

differently. In this case, the LT8500 sequentially pulses

PWM[1] through PWM[48] high for 64 PWMCK cycles

each. The state of the OPENLED pin is captured for each

channel while the corresponding PWM pin is high. This

by-channel data is shifted out in the status frame as the

nextframeisshiftedin.Inaddition,thestatusframewillset

the open LED test bit (OLT), indicating that the OPENLED

data in the current status frame is from the self test. The

statusframewillreturntotypicalreportingonthefollowing

frame. When the LT8500 is used with the LT3595A, the

OPENLEDpinandtheselftestprovideadiagnosticroutine

to identify the location of open LED faults. See “Diagnostic

InformationFlags”intheApplicationsInformationsection.

Figure 5. PWM Output Enable Tiꢁing Chart

Assuꢁes Outputs Were Previously Disabled

PHASE DIFFERENCE BETWEEN 16-CHANNEL BANKS

By default, the rising edges of all PWM[48:1] channels

occur on the same rising edge of PWMCK. This event

begins a PWM period of 4096 PWMCK cycles. The

LT8500 provides a phase-shift toggle command (CMD =

0x6X) to reduce system noise and current spikes result-

ing from 48 pins switching at once. The function of this

command is illustrated in Figure 6, case A. In phase-shift

mode, the PWM[48:1] outputs are divided into three

16-channel banks that are 120 degrees out-of-phase

with each other within a PWM period. This means that

channels PWM[48:33] will turn on with the rising edge of

PWMCK(1), then channels PWM[32:17] will turn on with

the rising edge of PWMCK(1365), 1/3 of the PWM period,

and channels PWM[16:1] will turn on with the rising edge

of PWMCK(2730), 2/3 of the PWM period.

OUTPUTS

After power-up or reset, no PWM[48:1] output will turn on

until an output enable frame is sent. The 12-bit PWMCK

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Table 1. Exaꢁple PWM Width Calculations (Base 10) with Correction Enabled (CRD = 0)

A

B

C

D

E

PWM UPDATE VALUE

SENT ON SDI

PRESCALED PWM

(A • 2/3)

CORRECTION REGISTER

(COR) VALUE

MULTIPLIER

(C + 32)/64

PWM WIDTH (B•D)

(IN UNITS OF t

)

PWMCK

3

2

63

32

0

1.484375

1.0

3

120

80

119

80

120

80

120

80

0.5

40

1200

4095

800

2730

63

32

0

1.484375

1.0

1188

800

400

4052

2730

1365

0.5

63

32

0

1.484375

1.0

0.5

PWM CALCULATION BY DIGITAL MULTIPLICATION OF

CORRECTION REGISTER AND PWM UPDATE VALUES

So, a correction multiplier of ~1.5 (CORn = 63) yields a

corrected PWM width of 4052 = 4095 • (2/3) • 1.484375.

ThePWM

widthisalwaysroundedtothenearestwhole

OUTn

The correction multiplier is used to automatically scale

the 12-bit PWM channel data before storing the PWM

update value for the respective channel. The correction

multiplier is disabled by the correction register disable bit

(CRD),whichistoggledbythecorrectiontogglecommand

(CMD=0x7X). When the correction multiplier is disabled,

the incoming data is stored unchanged:

number. Table 1 shows examples of PWM calculations for

selected register values. This means the maximum PWM

duty cycle with CRD=0 is 4052/4096, and with CRD=1 it

is 4095/4096.

COMMAND DESCRIPTIONS

The LT8500 implements eight commands, outlined in

Table 2. The commands (CMD) are encoded in the eight

LSB’s of a command frame, and so reside in the eight

LSB’s of the shift register when a frame has been com-

pletely shifted in. The command field is executed by the

rising edge of LDI. Only the four MSB’s of the command

field are decoded for commands.

PWM

= CHAN

n(NOM)

OUTn

The correction multiplier is enabled by default (CRD=0)

and scales incoming channel data according to:

2

 

3

DCR + 32





 

n

GSR_n(CALC)= GSR_{n (NOM)}

•

 









64

where PWM

is the number of PWMCK cycles that

OUTn

PWMn is high, CHAN

is the nth channel field in

Synchronous Update Fraꢁe: CMD = 0x0X

n(NOM)

the frame, and CORn is the nth programmed correction

setting (CORn = 0 to 63). See Table 1 for examples.

AsynchronousupdateframeupdatesPWM[48:1]withthe

data in the frame, after processing through the Correction

Multiplier. The PWMR is updated when LDIBLANK goes

high. The PWMRSYNC register will be written from the

PWMR synchronously to the start of the PWM period (on

PWMCK 1). This command eliminates shortened PWM

“runt” pulses. The value in the PWMRSYNC registers

will update the PWM outputs on the next rising edge of

PWMCK. Examples are shown in Figure 6, cases B and E.

The 6-bit COR value sets a multiplier of 0.5X to ~1.5X

(exactly 1.484375, or ((63 + 32)/64)) with 64 values and

a midrange, signifying a multiple of 1.0, at 32 (0x20). In

order to avoid overflow in the PWM registers when the

multiplier is greater than 1.0, the nominal PWM update

value (CHANn) is first prescaled on chip by 2/3. This

means that the full-scale width for a channel with a mul-

tiplier of 1.0 (CHANn = 4095, CORn = 32) will result in a

PWM

width of 4095 • (2/3) • 1.0 = 2730, not 4095.

OUTn

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Table 2. Coꢁꢁand Register Decoding

CMD (CR[7:0])

0000_xxxx

0001_xxxx

0010_xxxx

0011_xxxx

0100_xxxx

0101_xxxx

0110_xxxx

0111_xxxx

1xxx_xxxx

NAME

SUMMARY

FRAME DATA

Synchronous Update Frame

Asynchronous Update Frame

Correction Frame

Update PWM’s Synchronously to PWM Period

Update PWM’s Asynchronously to PWM Period

Set PWM Correction Factor

PWM Update by Channel

Correction by Channel

Don’t Care

Output Enable Frame

Output Disable Frame

Self Test Frame

Enable PWM Outputs

Disable (Drive Low) PWM Outputs

Initiates Self Test

Don’t Care

Phase-Shift Toggle Frame

Correction Toggle Frame

Reserved

Toggle 16-Channel Bank 120° Phase-Shift (PHS)

Toggle Correction Disable Bit in Multiplier (CRD)

Do Not Use

Don’t Care

–

Asynchronous Update Fraꢁe: CMD = 0x1X

Output Disable Fraꢁe: CMD = 0x4X

An asynchronous update frame updates PWM[48:1] with

the data in the frame, after processing through the correc-

tionmultiplier.ThePWMRisupdatedwhenLDIBLANKgoes

high.ThePWMRSYNCregisterwillbewrittenimmediately

(asynchronously), through the PWMR, when LDI is high.

ThevalueinthePWMRSYNCregisterswillupdatethePWM

outputs on the next rising edge of PWMCK. Examples are

shown in Figure 6, cases C and F.

An output disable frame immediately resets the PWMCK

counterwhenLDIgoeshigh,anddisablesthePWMoutputs

on the next rising edge of PWMCK. There is no effect on

either SDO or SCKO. The data in the output disable frame

is irrelevant to the command, but allows a daisy chain of

LT8500’s to function properly.

Self Test Fraꢁe: CMD = 0x5X

The self test frame can be used for diagnostics on each

PWM[48:1], including identifying open LED strings on

an LT3595A. After LDIBLANK goes hi, the LT8500 pulses

PWM[1] through PWM[48] sequentially for 64 PWMCK

cycles each. The state of the OPENLED pin is captured

for each channel while the corresponding PWM pin is

high. This by-channel data is subsequently shifted out in

the status frame. In addition, the status frame will set the

open LED test bit (OLT) to confirm that the OPENLED data

in the current status frame is from the self test. For all

othercommands,thestateoftheOPENLEDpiniscaptured

once on the first SCKI of the frame. The same value is

then reported in the status frame on all 48 channels. The

data in the self test frame is irrelevant to the command,

but allows a daisy chain of LT8500’s to function properly.

Correction Fraꢁe: CMD = 0x2X

A correction frame updates the correction registers (COR)

with the six MSB’s of each channel’s data field in the

frame. The CORs are used by the correction multiplier to

adjust the PWM width, prescaled by 2/3, by a multiplier of

between 0.5 and ~1.5. Example PWM width calculations

are shown in Table 1. In typical applications, this com-

mand will only be run once after power-up to initialize the

system. Therefore, a correction frame will not update the

PWM outputs. The update frame that follows a correction

frame will reflect the COR update.

Output Enable Fraꢁe: CMD = 0x3X

An output enable frame starts a PWM period, and enables

the PWM outputs, on the second PWMCK edge after

LDIBLANK goes high. There is no effect on either SDO or

SCKO. The data in the output enable frame is irrelevant

to the command, but allows a daisy chain of LT8500’s to

function properly.

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Phase-Shift Toggle Fraꢁe: CMD = 0x6X

Case B illustrates a synchronous update frame (CMD =

0x0X) while in phase-shift mode, as in case A. The LDI

signal goes active 512 PWMCK cycles into the PWM

period, after PWM[48] has turned off. The update frame

programs a PWM width of 1024, but the synchronous

update command prevents a channel from updating

except at the beginning of its PWM period. As a result,

PWM[48] remains low until the next PWM period, when

the updated width drives it high for 1024 PWMCK cycles.

PWM[32] begins its PWM period at PWMCK 1365, and

PWM[16] starts at PWMCK 2730, both updated to 1024

PWMCK cycles.

Thephase-shifttoggleframetogglesthephase-shift(PHS)

bit, which is off by default. When PHS is set, it sets the

rising edges of the PWM outputs, by banks of 16 chan-

nels, out-of-phase with each other by 120 degrees. This

meansthatchannelsPWM[48:33]willstartthePWMcycle

with a rising edge at the beginning of a PWM period, then

channels PWM[32:17] will start their PWM cycle 1/3 of

the time into a PWM period, and channels PWM[16:1] will

start 2/3 of the time into a PWM period. The state of the

PHS bit is returned in every status frame. The data in the

phase-shift toggle frame is irrelevant to the command,

but allows a daisy chain of LT8500’s to function properly.

Case C illustrates an asynchronous update frame (CMD

= 0x1X) while in phase-shift mode, as in case A. The LDI

signalgoesactive512PWMCKcyclesintothePWMperiod,

afterPWM[48]hasturnedoff. Theupdateframeprograms

a PWM width of 1024, and because it is an asynchronous

update, PWM[48] immediately rises and stays high until

PWMCK 1024. PWM[32] and PWM[16] (and all PWM’s)

are also updated, but no rising edge occurs until their

PWM period begins due to the phase-shifting.

Correction Toggle Fraꢁe: CMD = 0x7X

Thecorrectiontoggleframetogglesthecorrectionregister

disable(CRD)bit, whichisoffbydefault. WhenCRDisset,

it disables use of the correction registers (CORs) in the

correctionmultiplier,insteadmultiplyingtheincomingdata

from SDI by “1.” This causes the data in an update frame

to reach the PWMRSYNC registers unchanged. The state

oftheCRDbitisreturnedineverystatusframe. Thedatain

the correction toggle frame is irrelevant to the command,

but allows a daisy chain of LT8500’s to function properly.

Case D illustrates the default (not phase-shifted) mode in

steady-state. All PWM outputs rise on the same PWMCK

edge at the beginning of the PWM period.

Case E illustrates a synchronous update frame (CMD =

0x0X) without phase-shifting, as in case D. The LDI signal

goes active 512 PWMCK cycles into the PWM period, after

the PWMs have turned off. The update programs a PWM

width of 1024, but the synchronous update command

prevents a channel from updating except at the beginning

of it’s PWM period. As a result, all PWM’s remain low until

thenextPWMperiod, whentheupdatedwidthdrivesthem

high for 1024 PWMCK cycles.

Exaꢁples of PWM Updates for Selected Cases

Figure6showsexamplesoftheeffectofvariouscommands

onthePWMoutputwaveforms.Theseexamplewaveforms

assume all three channels shown are always programmed

for the same PWM width. For each case, a representative

channelisshownfromeachofthethree16channelbanks,

PWM[48:33], PWM[32:17], and PWM[16:1].

CaseAillustratesthephase-shiftmodeinsteady-state,with

PWM’s programmed for a width of 256 PWMCK cycles.

PWM[48], from bank 2, rises at the beginning of the PWM

period. PWM[32], from bank 1, rises 1/3 of the way into

the PWM period of bank 2, or 1365 PWMCK cycles later.

PWM[16], from bank 0, rises 2/3 of the way into the PWM

period of bank 2, or 2730 PWMCK cycles later.

Case F illustrates an asynchronous update frame (CMD =

0x1X) without phase-shifting, as in case D. The LDI signal

goes active 512. PWMCK cycles into the PWM period,

after the PWMs have turned off. The update programs a

PWM width of 1024, and because it is an asynchronous

update, all PWM’s immediately rise and stay high until

PWMCK 1024.

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4096 • t

PWMCK

2730 • t

PWMCK

1365 • t

PWMCK

256 • t

PWMCK

PWM [48]

PWM [32]

PWM [16]

LDI

CASE A: STEADY STATE WITH PHASE-SHIFT

512 • t

PWMCK

PWM [48]

PWM [32]

PWM [16]

CASE B: SYNCHRONOUS UPDATE WITH PHASE-SHIFT

CASE C: ASYNCHRONOUS UPDATE WITH PHASE-SHIFT

1024 • t

PWMCK

PWM [48]

PWM [32]

PWM [16]

PWM [48]

PWM [32]

PWM [16]

CASE D: DEFAULT STEADY STATE (NO PHASE-SHIFT)

LDI

PWM [48]

PWM [32]

PWM [16]

CASE E: SYNCHRONOUS UPDATE (NO PHASE-SHIFT)

PWM [48]

PWM [32]

PWM [16]

8500 F06

CASE F: ASYNCHRONOUS UPDATE (NO PHASE-SHIFT)

Figure 6. Exaꢁples of PWM Outputs For Selected Coꢁꢁand Cases

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applicaTions inForMaTion

This section is illustrated with an LED dimming applica-

tion, but is relevant to other applications as well. The

LT8500 provides 48 PWM outputs, such as for driving

three LT3595A LED drivers. The LT8500 provides an LED

dot correction function using digital multiplication of the

correction register (COR) and the PWM update value,

which is prescaled by 2/3. This results in a dot corrected

PWM duty cycle. Optionally, the PWM update can be

written directly (unchanged) by setting the correction

register disable bit (CMD = 0x7X). When this bit is set,

the multiplication is bypassed and dot correction, if any,

must be calculated off-chip. The PWM duty cycle in this

case will be the nominal value sent in the update frame,

divided by 4096. The part provides a status frame with

OPENLEDandCORdataforeachchannel, andglobalstate

data indicating self testing (such as for open LED’s), out-

of-sync error, phase-shift status, and direct data status.

The status frame is shifted out of the part whenever a new

frame is shifted in. An on-chip self test is available (CMD

= 0x5X) to determine which channel is responsible for a

fault, such as open LEDs. The OPENLED pin and self test

are especially suited for use with the LT3595A. In this

application, the self test will identify which channels have

opens in their LED strings. This Applications Information

section serves as a guideline for avoiding common pitfalls

for the typical application.

where GSR

(same as PWMR) setting (GSR

dot correction enabled).

is the nth calculated grayscale register

n(CALC)

= 0 to 4052 with

n(CALC)

Setting Dot Correction

The LT8500 can adjust the PWM duty cycle for each chan-

nel independently. The duty cycle adjustment, also called

dot correction, is mainly used to calibrate the brightness

deviationbetweenLED channels. The 6-bit(64 values) dot

correction registers (DCR) adjust each PWM duty cycle

from 0.5X to ~1.5X of the duty cycle, prescaled by 2/3,

sent to the grayscale register (GSR) according to

2

 

3

DCR + 32





 

n

GSR_n(CALC)= GSR_{n (NOM)}

•

 









64

whereGSR

isthenthcalculatedgrayscaleregister,

n(CALC)

GSR

isthenominalgrayscalevaluesenttothenthchan-

n(NOM)

nelandDCR isthenthprogrammeddotcorrectionsetting

n

(DCR = 0 to 63).

n

Cascading Devices and Deterꢁining Serial Data

Interface Clock

In a large LCD backlighting or LED display system, mul-

tiple LT8500 chips can be easily cascaded to drive all LED

drivers, such as the LT3595A, and their associated LED

strings.TheLT8500adoptsanovel5-wiretopology,which

balances clock skew and eases PCB layout.

Setting Grayscale by PWM Updates

Although adjusting the LED current changes its luminous

intensity, or brightness, it will also affect the color match-

ing between LED channels by shifting the chromaticity

coordinate. The best way to adjust the brightness is to

control the amount of LED on/off time by pulse width

modulation (PWM).

The time required to send a set of cascaded frames is 584

SCKI cycles per LT8500, plus another cycle time for LDI.

Assuming LDI is externally balanced, the minimum serial

data interface clock frequency ƒ

system can be calculated as:

for a large display

SCK

ƒ

SCK

= [(n • 584) + 1] • ƒ

CHIPS REFRESH

The LT8500 can adjust the brightness for each channel

independently. The 12-bit PWM registers (PWMR), used

for grayscale (GS) dimming, results in 4095 different

brightness steps from 0ꢀ to 99.98ꢀ. The brightness

level, or PWM duty cycle, GSnꢀ for channel n can be

calculated as:

where n

REFRESH

is the number of cascaded LT8500s and

CHIPS

ƒ

is the refresh rate of the whole system.

Status Fraꢁe Inforꢁation

The status frame is captured and shifted out of SDO as a

new data frame shifts in on SDI. The format of a status

frame is shown in Figure 3. With the exception of the

diagnostic flags (SYC and NOL[48:1]), the data in the

status frame does not change without a command from

8500fb

GSR

GSnꢀ=

^n(CALC)•100ꢀ

4096

18

For more information www.linear.com/LT8500

LT8500

applicaTions inForMaTion

the user interface. It can therefore be monitored to con-

firm proper communication with the chip. The following

non-diagnostic status information is continually provided

in the status frame: dot correct registers for each channel

(COR[48:1]), Open LED Testing bit (OLT), phase-shift bit

(PHS), correction register disable (CRD) bit. There are

five unused bits, [5:1], in the field associated with each

channel, all of which are always set to logic zero.

open, each of the 48 OPENLED (NOL[48:1]) status flags

will be cleared. Upon detecting this condition in the status

frame, or as a polling strategy, the host may request an

LED self test (CMD = 0x5X), where the LT8500 will test

each channel to determine which, if any, is open. The test

drives each PWM pin high, one at a time, in order, for 64

PWMCKcycleseach,andcapturesthecorrespondingvalue

on the OPENLED pin for the associated PWM channel.

These results will overwrite the NOL flags in the status

frame and the open LED test bit (OLT) will be set in the

status frame to indicate that the NOL data in this status

frame is given by channel. In the next frame, the OLT bit

will be cleared and all 48 NOL bits will again reflect the

state of the OPENLED pin.

Diagnostic Inforꢁation Flags

The LT8500 features two kinds of diagnostic information

flags: global out-of-sync error (SYC) and 48 individual

open LED flags (NOL[48:1]).

An out-of-sync error occurs when the part sees an LDI

signal unexpectedly, whether before 584 SCKI clocks,

or coinciding with SCKI high. Either of these events can

corrupt the data and the state of the chip. The SYC bit is

available in every status frame to notify the system if an

erroneous LDI was seen since the first rising edge of SCKI

ofthelastframe.AseriesofmultipleLDI’sbetweenframes,

with no SCKI, is not an out-of-sync error. Recovery from

an out-of-sync error may require the user to completely

rewrite the data and state of the chip. The LDI signal resets

the serial interface.

PCB Layout Guidelines

The following guidelines should be considered when de-

signing printed circuit boards (PCBs) using the LT8500.

These guidelines are more important as clock speeds and

daisy chain sizes increase.

1. Match the line lengths and delays between SDI and

SCKI to each LT8500.

2. Ensure the timing of LDI to each chip meets SCKI to LDI

setup and hold requirements. In a 5-pin topology, SCKI

is delayed by each chip in the daisy chain, so LDI may

need extra delay to match the delayed SCKI down the

chain. See the discussion on topology in the Operation

section.

The OPENLED bits, NOL[48:1], are well suited for use

with the LT3595A, and indicate an open circuit has been

detected on at least one of the 48 LED strings driven

by the three LT3595A’s. The part monitors the three

LT3595A wired-OR OPENLED pins that detect open LED

strings for each LT3595A. When one of the LT3595A’s

detects an open LED string, it will pull OPENLED low dur-

ing the PWM high time for that LED string. The state of

OPENLED is captured by the LT8500 on the rising edge

of the first SCKI of a new frame (after LDI). Since SCKI

and PWMCK are asynchronous, the detection of an

open LED string by this method is a probability function

dependent on the frame rate and PWM duty cycle. If a new

frame begins when the PWM pin associated with an open

LED string is high, the OPENLED pin will be driven low and

captured in the status register, but if a new frame begins

whentheassociatedPWMpinislow,theOPENLEDpinwill

be pulled high and the status register will capture a default

high. When a low OPENLED pin is captured, signaling an

3. Avoid cross talk between the communication signals

(SDI, SCKI, LDI, SDO, SCKO) and the PWMs. Even

though the PWM’s signals toggle at a slow rate, all of

their rising edges can occur within a few nanoseconds

of each other.

4. Buffer the signals returning to the host if their paths

are long.

5. Highspeedtechniques:standardhighspeedPCBdesign

techniquesshouldbeusedonhighfrequencyclockand

datalines.Theseincludeshortpathlengths,shieldingof

high speed data cables and traces, minimized parasitic

capacitance, and reducing antennas and reflections.

6. A ceramic bypass capacitor should be placed close to

the V pin.

CC

8500fb

19

For more information www.linear.com/LT8500

LT8500

Typical applicaTions

Four typical applications are shown in Figures 7 to 10.

Figures 7 and 8 illustrate the 5-pin and 4-pin topologies

for daisy chains as discussed earlier in this data sheet.

Figure 9 illustrates a single LT8500 controlling 48 resistor

ballasted LED strings. Figure 10 illustrates a novel use of

the LT8500 as a 48-channel digital-to-analog converter

(DAC). Using a simple RC filter on each PWM output, the

resulting converter has very good error characteristics

as shown in the accompanying differential linearity error

(DLE) and integrated linearity error (ILE) charts (Figures

11 and 12). The DLE measurements were taken from an

all codes test, and were compensated for power supply

variation on V of less than 0.01ꢀ over the course of

CC

the test. The ILE is simply the sum of all previous com-

pensated DLE measurements. The units of the DLE and

ILE measurements are in PWM LSB’s.

8500fb

20

For more information www.linear.com/LT8500

LT8500

Typical applicaTions

8500fb

21

For more information www.linear.com/LT8500

LT8500

Typical applicaTions

V

DD

12V

HOST

V

DD

3.0V TO 5.5V

SCKI

SDI

LDIBLANK

PWM1

M1

V

CC

•

LT8500

PWMCK

OK TO FLOAT

PWMCK

OPENLED

CMLDM7003

M48

PWM48

SCKO

SDO

GND

8500 TA04

OK TO

FLOAT

Figure 9. Single LT8500 Driving 48 Resistor Ballasted LED Strings Froꢁ a V_DDRail

HOST

3.0V TO 5.5V

SCKI

SDI

LDIBLANK

PWM1

100k

ANALOG OUT1

ANALOG OUT48

V

CC

•

1µF

LT8500

PWMCK

OK TO FLOAT

PWMCK

OPENLED

PWM48

SCKO

SDO

GND

8500 TA05

OK TO

FLOAT

Figure 10. Single LT8500 Iꢁpleꢁenting 48 Digital-to-Analog Converter (DAC) Channels

1.0

0.8

1.0

0.8

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

–0.8

–1.0

–0.2

–0.4

–0.6

–0.8

–1.0

0

512 1024 1536 2048 2560 3072 3584 4096

0

512 1024 1536 2048 2560 3072 3584 4096

PWM WIDTH CODE

8500 TA06

8500 TA07

Figure 11. DAC Differential Linearity Error (DLE)

Figure 12. DAC Integrated Linearity Error (ILE)

8500fb

22

For more information www.linear.com/LT8500

LT8500

package DescripTion

Please refer to http://www.linear.coꢁ/product/LT8500#packaging for the ꢁost recent package drawings.

TJ Package

56-Lead Plastic TLA QFN (6mm × 6mm)

(Reference LTC DWG # 05-08-1968 Rev C)

0.15 0.05

0.50 0.05

3.60 0.05

0.75

6.30 0.05

5.30 0.05

4.70 0.05

4.00 0.05

0.55 0.05

3.10

0.05

0.6

0.125

PACKAGE OUTLINE

0.125

0.30 0.05

0.80 0.05

0.15 0.05

0.55 0.05

5.20 0.05

6.30 0.05

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.78 0.05

6.00 0.10

(4 SIDES)

0.30 0.05

A31

A40 A1

PIN 1 TOP MARK

(SEE NOTE 6)

0.45 REF

B16

B1

PIN 1 NOTCH

R = 0.45 OR

0.35 × 45°

CHAMFER

3.60 0.10

0.30 0.05

0.55

BSC

3.20 0.10

B8

B9

0.8 REF

0.10 REF

A11

A21

0.25 REF

0.03–0.09

0.55 BSC

0.30 0.05

NOTE:

0.10 REF

1. DRAWING NOT TO SCALE

(TL56) QFN REV C 0815

2. ALL DIMENSIONS ARE IN MILLIMETERS

BOTTOM VIEW—EXPOSED PAD

3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE, IF PRESENT

4. EXPOSED PAD SHALL BE SOLDER PLATED

5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE

6. 4 MIL THICK LASER CUT STENCIL IS RECOMMENDED. STENCIL OPENING 1:1 TO PCB LAND PATTERN.

8500fb

23

For more information www.linear.com/LT8500

LT8500

package DescripTion

Please refer to http://www.linear.coꢁ/product/LT8500#packaging for the ꢁost recent package drawings.

Not Recoꢁꢁended for New Designs

UHH Package

56-Lead Plastic QFN (5mm × 9mm)

(Reference LTC DWG # 05-08-1727 Rev B)

0.70 ±0.05

5.50 ±0.05

(2 SIDES)

4.10 ±0.05

(2 SIDES)

3.60 REF

3.45 ±0.05

(2 SIDES)

7.13 ±0.05

PACKAGE

OUTLINE

0.20 ±0.05

0.40 BSC

6.80 REF (2 SIDES)

8.10 ±0.05 (2 SIDES)

9.50 ±0.05 (2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

PIN 1 NOTCH

R = 0.30 TYP OR

0.35 × 45° CHAMFER

0.75 ±0.05

5.00 ±0.10

3.60 REF

(2 SIDES)

55 56

0.00 – 0.05

0.40 ±0.10

PIN 1

TOP MARK

1

2

(SEE NOTE 6)

9.00 ±0.10

(2 SIDES)

6.80 REF

7.13 ±0.10

3.45 ±0.10

(UHH) QFN 0515 REV B

0.200 REF

0.20 ±0.05

0.40 BSC

BOTTOM VIEW—EXPOSED PAD

R = 0.10

TYP

0.200 REF

0.00 – 0.05

0.75 ±0.05

NOTE:

1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE

2. DRAWING NOT TO SCALE

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

3. ALL DIMENSIONS ARE IN MILLIMETERS

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

8500fb

24

For more information www.linear.com/LT8500

LT8500

revision hisTory

REV

DATE

12/15 Added TJ Package.

Clarified Pin Functions to include TJ Package.

DESCRIPTION

PAGE NUMBER

A

2, 23

6

Added UHH Package Not Recommended for New Designs.

06/16 Clarified Setting Dot Correction section

24

18

B

Clarified Status Frame Information section pin functions to include TJ package

8500fb

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

For more information www.linear.com/LT8500

25

LT8500

Typical applicaTion

Single LT8500 Driving 48 Resistor Ballasted LED Strings Froꢁ a V_DDRail

V

DD

12V

HOST

V

DD

3.0V TO 5.5V

SCKI

SDI

LDIBLANK

PWM1

M1

V

CC

•

LT8500

PWMCK

OK TO FLOAT

PWMCK

OPENLED

CMLDM7003

M48

PWM48

SCKO

SDO

GND

8500 TA08

OK TO

FLOAT

relaTeD parTs

PART NUMBER DESCRIPTION

COMMENTS

= 6V, V

LT3746

55V, 1MHz 32-Channel Full Featured 30mA Step-Down LED Driver V

= 55V, V

= 13V, Dimming = 5,000:1

IN(MIN)

IN(MAX)

SD

OUT(MAX)

True Color PWM, I < 1µA, Package 5mm × 9mm QFN-56

LT3595/

LT3595A

45V, 2.5MHz 16-Channel, 50mA Full Featured Boost LED Driver

V

= 4.5V, V

= 45V, V

SD

= 45V, Dimming =

IN(MIN)

IN(MAX)

OUT(MAX)

5,000:1 True Color PWM, I < 1µA, Package 5mm × 9mm QFN-56

LT3754

LT3598

LT3599

60V, 1MHz Boost 16-Channel, 50mA LED Driver with True Color

3,000:1 PWM Dimming and 2ꢀ Current Matching

V

= 4.5V, V

= 40V, V

SD

= 60V, Dimming =

IN(MIN)

IN(MAX)

OUT(MAX)

3,000:1 True Color PWM, I < 1µA, Package 5mm × 5mm QFN-32

44V, 1.5A, 2.5MHz Boost 6-Channel, 30mA LED Driver

V

= 3V, V

= 30V(40V

SD

), V

= 44V, Dimming =

IN(MIN)

IN(MAX)

MAX

OUT(MAX)

1,000:1 True Color PWM, I < 1µA, Package 4mm × 4mm QFN-24

44V, 2A, 2.5MHz Boost 4-Channel, 120mA LED Driver

V

= 3V, V

= 30V(40V

SD

), V

MAX

= 44V, Dimming =

IN(MIN)

IN(MAX)

OUT(MAX)

1,000:1 True Color PWM, I < 1µA, Package 4mm × 4mm QFN-24

8500fb

LT 0616 REV B • PRINTED IN USA

26 ^{LinearTechnology Corporation}

1630 McCarthy Blvd., Milpitas, CA 95035-7417

For more information www.linear.com/LT8500

●

 LINEAR TECHNOLOGY CORPORATION 2011

(408)432-1900 FAX: (408) 434-0507 www.linear.com/LT8500


型号：	LT8500ETJ#TRPBF
厂家：	Linear
描述：	LT8500 - 48-Channel LED PWM Generator with 12-Bit Resolution and 50MHz Cascadable Serial Interface; Package: QFN; Pins: 56; Temperature Range: -40°C to 85°C
文件：	总26页 (文件大小：336K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT8500ETJ#TRPBF [Linear]

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