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LTC2000

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OPERATION

Measuring Internal Junction Temperature (TJ)

The LTC2000 test multiplexer may also be used to con-

nect internal junction temperature measurement diodes

to the TSTP/N pins. Ensure LMX_EN = 0 (register address

0x18) and use SPI register 0x19 to set TDIO_EN = 1 to

enable this function (Table 9). There are two methods the

user can choose from to measure internal junction tem-

perature (TJ). For TDIO_SEL = 0, an unbiased NPN tran-

sistor is diode-connected between the TSTP/N pins with

a series resistance of approximately 350Ω. This diode is

suitable for use with external temperature sensors which

offer series resistance cancellation such as the LTC2991

or LTC2997.

If such a temperature sensor is not available, set

TDIO_SEL = 1 to directly observe a temperature dependent

voltage between TSTP and TSTN. The typical expected

voltage at TSTP is VTSTP = 2.02V – 5.5mV/°C • (TJ – 25°C).

The junction temperature can be calculated as TJ = 25°C +

(2.02V – VTSTP)/(5.5mV/°C). For best accuracy with

TDIO_SEL = 1, use TSTN to sense GND at the bottom of

the diode and calibrate the voltage at a known tempera-

ture. Typical uncalibrated accuracy is ±5°C.

Pattern Generator

A 64 sample deep pattern generator is included in the

LTC2000 to simplify system development and debug. The

pattern generator allows the user to send a repeating 64

sample pattern to the DAC, completely independent of the

presence or absence of valid signals on DCKIP/N, DAP/N,

and DBP/N.

Table 9. Internal Junction Temperature SPI Registers

ADDRESS

BIT

NAME

DESCRIPTION

0x19

TDIO_EN

TSTP/N Junction Temperature Diode Enable. Set TDIO_EN = 1 to measure internal junction temperature (TJ) at TSTP/N.

Ensure LMX_EN = 0 when TDIO_EN = 1.

TDIO_SEL Selects which internal temperature diode is observable at TSTP/N.

For TDIO_SEL = 1, the typical voltage at TSTP with respect to TSTN is VTSTP = 2.02V – 5.5mV/°C • (TJ – 25°C).

Junction temperature can be calculated as TJ = 25°C + (2.02V – VTSTP)/(5.5mV/°C). Typical accuracy is ±5°C.

For TDIO_SEL = 0, an unbiased diode is connected b/w TSTP/N for use with external temperature sensors.

Note: Register 0x19 resets to 0x00 (default).

To use this feature, do the following:

1. Set DCKO_DIV = 0 in register 0x02, DATA_SP = 0 and

DATA_EN = 0 in register 0x04, and PGEN_EN = 0 in

2. Write 128 bytes of pattern data to address 0x1F

(PGEN_D) to fill the pattern generator with 64 sam-

ples. Data is written MSB first, and will be applied to

the DAC in the order written. Data may be written one

byte at a time or in larger multi-byte words. For the

LTC2000-14 and LTC2000-11, data should be left justi-

fied with zeros filling the remaining two (LTC2000-14)

or five (LTC2000-11) bits.

3. Set PGEN_EN = 1 to start the pattern generator.

4. Wait at least 1ms to ensure that the synchronizer has

initialized.

5. Set DATA_EN = 1 in register 0x04. The DAC will then

begin to output the 64 sample pattern.

The pattern generator will send the repeating 64 sample

pattern to the DAC until the user writes PGEN_EN = 0 or

DATA_EN = 0.

To read back the pattern, set DATA_EN = 0 and PGEN_EN =

0 and then read 128 bytes from address 0x1F. Note that the

starting point of the pattern may have changed while the

pattern was running. To modify the pattern, set DATA_EN =

0 and PGEN_EN = 0 and write a new 64 sample pattern to

address 0x1F. Ensure PGEN_EN = 0 when reading or writ-

ing to address 0x1F, and always read or write an entire 64

sample pattern prior to setting PGEN_EN = 1. See Table 10.

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