LTC1695 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1695' PDF : 20 Pages View PDF
LTC1695
APPLICATIONS INFORMATION
VCC
VCC/2
64 RESISTOR
VOLTAGE TABS 720
SWITCHES
REFERENCE
OP AMP
“000000” = 0V
“111111” = 0.984 • VCC/2
6
GND
SMBus
COMMAND
D5 to D0
1695 • F03
Figure 3. Ladder DAC
DAC
The LTC1695 uses a 128-segment resistor ladder to
implement the monotonic 6-bit voltage DAC (Figure 3).
Guaranteeing monotonicity (no missing codes) permits
the use of the LTC1695 in thermal feedback control
applications. As the typical application uses a 5V supply
for VCC, the reference for the 6-bit DAC is VCC. LTC
recommends a 10µF or greater tantalum capacitor to
bypass VCC. Users must account for the variation in the
DAC’s output absolute accuracy as VCC varies. VCC voltage
should not exceed the absolute maximum rating of 7V or
drop below the typical 2.8V undervoltage lockout thresh-
old (UVLO) during normal operation.
The LTC1695’s DAC specifications (INL, DNL, VOS) ac-
count for the offset and gain errors of the linear regulator
with respect to ILOAD. Consult the Definitions section for
more details.
The worst-case condition occurs if the LTC1695 P-chan-
nel pass transistor enters dropout at full-scale VOUT and
ILOAD. Full-scale VOUT (VFS) is 4.922V with VCC = 5V. In this
condition, loop gain drops and gain error increases. The
LTC1695 is designed for monotonicity up to VFS with DNL
and INL less than 0.75 LSB. Refer to the Electrical Char-
acteristics and Typical Performance Characteristics for
more information.
Linear Regulator Loop Compensation
The LTC1695’s linear regulator approach is a simple and
practical scheme for fan speed control featuring a wide and
linear dynamic range. It also introduces less noise into the
system supply rail, compared with a PWM scheme (fixed
frequency, variable duty cycle), switching regulator topol-
ogy or simple ON-OFF control.
The LTC1695 linear regulator feedback loop requires a
capacitor at its output to stabilize the loop over the output
voltage and load current range. The output capacitor value
and the capacitor’s ESR value are critical in stabilizing the
LTC1695 feedback loop.
A ≥ 1µF general purpose, low to medium ESR (0.1Ω to 5Ω)
tantalum or aluminium electrolytic capacitor is sufficient
for most applications. These capacitor types offer a low-
cost advantage, particularly for fan speed control applica-
tions. As the output capacitance value increases, stability
improves. A typical 4.7µF, 1Ω ESR surface mount tanta-
lum capacitor is recommended for the optimum transient
response and frequency stability across temperature, VOUT
and ILOAD. Refer to the load transient response waveforms
in the Typical Performance Characteristics section.
The selection of the capacitor for COUT must be evaluated
by the user for temperature variation of the capacitance
and ESR value and the voltage coefficient of the capacitor
value. For example, the ESR of aluminium electrolytic
capacitors can increase dramatically at cold temperature.
Therefore, the regulator may be stable at room tempera-
ture but oscillate at cold temperature. Ceramic capacitors
with Z5U and Y5 dielectrics provide high capacitance
values in a small package, but exhibit strong voltage and
temperature coefficients (–80% in some cases). In addi-
tion, the ESR of surface mount ceramic capacitors is too
low (<0.1Ω) to provide adequate phase-lead in the feed-
back loop for stability.
Fan Load and CLOAD
Referring to Figure 4, CLOAD varies greatly depending on
the type of fan used. The simplest, inexpensive fans
contain no protection circuitry and input capacitance is on
the order of 200pF. More expensive fans generally incor-
porate a series-diode for reverse protection and input
13
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