Datasheet

LTC2991
14
2991fd
For more information www.linear.com/LTC2991
applicaTions inForMaTion
changes in temperature using the approximate relation-
ship of –2.1mV/°C of voltage dependence on temperature.
With an
LSB weight of 38.15µV and a diode temperature
relationship of –2.1mV/°C this yields ~0.018 degree resolu
-
tion. For sensor applications involving heaters, the ability
to sense small changes in temperature with low noise
can yield significant power savings, allowing the heater
power to be reduced. Table 16 has some conversion result
examples for various diode voltages.
Voltage/Current
Voltage results are reported in two respective registers,
an MSB and LSB register. The Voltage MSB result register
most significant bit (bit 7) is the DATA_VALID bit, which
indicates whether the current register contents have been
accessed since the result was written to the register. This
bit will be set when the register contents are new, and
cleared when accessed. Bit 6 of the MSB register is the
sign bit, bits 5 though 0 represent bits D[13:8] of the
two’s complement conversion result. The LSB register
holds conversion bits D[7:0]. The LSB value is different
for single-ended voltage measurements V1 through V8,
and differential (current measurements) V1 – V2 , V3 – V4,
V5 – V6 and V7 – V8. Single-ended voltages are limited
to positive values in the range 0V to 4.9V or V
CC
+ 0.2V,
whichever is smaller. Differential voltages can have input
values in the range of –0.300V to 0.300V.
Use the following equations to convert the register values
(see Table 16 for examples):
V
SINGLE_ENDED
= D[13:0] • 305.18µV
V
DIFFERENTIAL
= D[13:0] • 19.075µV, if sign = 0
V
DIFFERENTIAL
= (D[13:0] +1) • –19.075µV, if sign = 1
Current = D[13:0] • 19.075µV/R
SENSE
, if sign = 0
Current = (D[13:0] +1) • –19.075µV/R
SENSE
, if sign = 1,
Where R
SENSE
is the current sensing resistor, typically
< 1Ω.
V
CC
The LTC2991 measures V
CC
. To convert the contents of
the V
CC
register to voltage, use the following equation:
V
CC
= 2.5 + (D[13:0] • 305.18µV).
PWM Output
A 9-bit, 1kHz PWM output proportional to temperature V7
is available for controlling fans or heaters. PWM_Thresh
-
old is a 9-bit value with an LSB weighting of one degree
Kelvin. P
WM_Threshold is subtracted from V7 and a
pulse width proportional to the difference is produced.
Note that the PWM threshold is split among two regis
-
ters, with PWM_Threshold[8:1] in register 09h[7:0] and
PWM_Threshold[0] in register 08h[7]. Equation 9 shows
the registers involved. The P
WM frequency is ~1kHz. The
PWM output can be disabled or inverted with the PWM
enable and PWM invert bits is register 08h, respectively.
Figure 9 illustrates the PWM transfer function. The equa
-
tion for the duty cycle is:
(9)
PWM_DUTY_CYCLE %
(
)
=
100 • (REG7 – PWM • 16)
512
Where REG7 is bits [12:0] and
PWM is PWM Threshold bits [8:0]
A 50% duty cycle PWM signal would occur, for example,
if the PWM threshold was set to 10h (16°C) and register
7 contained 200h (32°C). If channel 7 is configured for
Kelvin temperatures, the PWM threshold must also be a
Kelvin temperature. The registers are two’s compliment
numbers. When calculating the duty cycle above for
Celsius temperatures care should be taken to sign extend
the register 7 and PWM threshold values. For temperatures
below the PWM Threshold, the PWM output pin will be a
constant logic level 0. For temperatures 32 degrees above
Figure 9. PWM Transfer Function
REG7[12:4]-PWM_THRESHOLD[8:0]
PWM DC (%)
50%
16 32
2991 F09
0
0%
99.8%
PWM INVERT = LOGIC 0