OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LTC1695CS5#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTC1695
13
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 ≥ 1 µF 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, V
OUT
and I
LOAD
. Refer to the load transient response waveforms
in the Typical Performance Characteristics section.
The selection of the capacitor for C
OUT
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 C
LOAD
Referring to Figure 4, C
LOAD
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
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 V
CC,
the reference for the 6-bit DAC is V
CC
. LTC
recommends a 10µF or greater tantalum capacitor to
bypass V
CC
. Users must account for the variation in the
DAC’s output absolute accuracy as V
CC
varies. V
CC
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, V
OS
) ac-
count for the offset and gain errors of the linear regulator
with respect to I
LOAD
. 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 V
OUT
and
I
LOAD.
Full-scale V
OUT
(V
FS
) is 4.922V with V
CC
= 5V. In this
condition, loop gain drops and gain error increases. The
LTC1695 is designed for monotonicity up to V
FS
with DNL
and INL less than 0.75 LSB. Refer to the Electrical Char-
acteristics and Typical Performance Characteristics for
more information.
Figure 3. Ladder DAC
64 RESISTOR
VOLTAGE TABS
720
SWITCHES
V
CC
GND
REFERENCE
OP AMP
“000000” = 0V
“111111” = 0.984 • V
CC
/2
6
SMBus
COMMAND
D5 to D0
V
CC
/2
1695 • F03
APPLICATIONS INFORMATION
WUU
U
LTC1695
14
Thermal Considerations
The LTC1695’s power handling capability is limited by the
maximum rated junction temperature of 125°C. Power
dissipation (P
DISS
) consists of two components:
1. Output current multiplied by the input/output voltage
differential: (I
LOAD
)(V
CC
– V
OUT
), and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
CC
).
P
DISS
= (I
LOAD
)(V
CC
– V
OUT
) + (I
GND
)(V
CC
)
T
J
= P
DISS
• (θ
JA
)
The LTC1695 has active current limiting and thermal
shutdown circuitry for device protection during overload
or fault condition. For continuous overload conditions, do
not exceed the 125°C maximum junction temperature
T
J(MAX)
. Give careful consideration to all thermal resis-
tance sources from junction to ambient. Consider any
additional heat sources mounted in proximity to the
LTC1695. This is particularly relevant in applications
where the LTC1695’s output is loaded with a constant
I
LOAD
and V
OUT
is dynamically varied via the SMBus. At
lower DAC output voltage codes, the increased input-to-
output differential increases power dissipation if I
LOAD
does not decrease.
For the LTC1695’s 5-lead SOT-23 surface mount package,
heat sinking is accomplished by using the heat spreading
capabilities of the PC board and its copper traces (in
particular, the GND pin trace).
The following table lists measured thermal resistance
results for various size boards and copper areas. All
measurements were taken in still air on 3/32" FR-4 board
with one ounce copper.
Table 2. Measured Thermal Resistance (
θθ
θθ
θ
JA
)
Topside* Backside Board Area
2500mm
2
2500mm
2
2500mm
2
125°C/W
1000mm
2
2500mm
2
2500mm
2
125°C/W
225mm
2
2500mm
2
2500mm
2
130°C/W
100mm
2
2500mm
2
2500mm
2
135°C/W
50mm
2
2500mm
2
2500mm
2
150°C/W
*Device is mounted on topside
+
+
–
+
OP AMP
+
+
C
GATE
C
NODE
V
CC
V
OUT
L
FAN
C
FAN
C
OUT
ESR
R1
R2
GND
EQUIVALENT
DC FAN CIRCUIT
INTERNAL
DAC
OUTPUT
P1(0.75Ω)
1695 • F04
Figure 4. Regulator Feedback Loop
capacitance ranges from 2pF to 30pF. As previously
discussed, an output bypass capacitor is required to
stabilize the feedback loop. This output capacitor is in
parallel with the fan’s input capacitance and dominates the
total capacitance. Thus, stability is generally not affected
by the fan’s input capacitance. The output capacitor also
serves to filter the fan’s output ripple during commutation
of the fan’s motor.
POR and UVLO
Under start-up conditions, the LTC1695 performs a power
on reset (POR) function. The digital logic circuitry is
disabled and the regulator is held off. The SMBus com-
mand register (to the DAC’s input) and data register
(current limit and thermal shutdown status) are reset to
zero. The POR signal deactivates if V
CC
rises above 2.9V
typically. The LTC1695 is then allowed to communicate
with the SMBus host and drive the fan accordingly. Upon
exiting POR, the regulator’s output voltage is set to V
ZS
(code 0) until programmed by the SMBus host.
The LTC1695 enters UVLO if V
CC
falls below 2.8V typically.
Between 2.8V and 1V, the digital logic circuitry is disabled,
the command/data registers are cleared and the regulator
is shut down. In general, 100mV of hysteresis exists
between the UVLO and POR thresholds.
APPLICATIONS INFORMATION
WUU
U
Thermal Resistance
(Junction to Ambient)
Copper Area
LTC1695
15
scale (V
FS
) until junction temperature decreases to
approximately 105°C. This extended timer period is an
attempt to cool down the system and the LTC1695 by
running the fan at full speed. In most cases, such elevated
ambient temperatures require the fan to run at full speed
anyway. The remaining LTC1695’s functionality remains
unchanged.
Thermal Shutdown, Overcurrent
The LTC1695 shuts down the P-channel linear regulator if
die temperature exceeds 155°C typically. The thermal
shutdown circuitry employs about 30°C of hysteresis. As
previously mentioned, the LTC1695 sets bit 6 (THE) in the
SMBus data byte register HIGH during thermal shutdown
conditions. During a fault condition, the LTC1695’s SMBus
logic continues to operate so that the SMBus host can read
back the fault status data.
During an overload or short-circuit fault condition, the
LTC1695’s current-limit detector sets bit 7 (OCF) in the
SMBus data byte register HIGH and actively limits output
current to 390mA typically. This protects the LTC1695’s
P-channel pass transistor. Under dead short conditions
with V
OUT
= 0V, the LTC1695 also clamps the output
current. However, the increased power dissipation
(5V • 390mA = 1.95W) eventually forces the LTC1695 into
thermal shutdown. The LTC1695 will then thermally oscil-
late until the fault condition is removed.
During recovery from thermal shutdown (typically 125°C),
the LTC1695 automatically activates the boost start timer,
programming the fan voltage to full scale for 250ms
(T
BST_ST
), before switching to the user programmed out-
put voltage value. This again eliminates fan start-up prob-
lems if the thermal shutdown fault occurred while the fan
was previously operating at an output voltage below the
fan’s starting voltage. In addition, as discussed, the boost
start timer will keep V
OUT
at V
FS
for an extended time
period beyond T
BST_ST
until the LTC1695’s junction tem-
perature drops below 105°C.
The LTC1695’s protection features protect itself, the fan,
and more importantly alerts the SMBus host to any system
thermal management fault conditions.
For further information, refer to the Junction Temperature
Increase (above ambient temperature) vs I
LOAD
graph in
the Typical Performance Characteristics section. This
graph provides a fast and simple junction temperature
estimation with various V
OUT
(DAC code) and I
LOAD
combinations for a typical application.
Boost Start Timer
In general, a 5V brushless DC fan starts at a voltage value
higher than the voltage at which it stalls. This behavior is
directly attributed to the force necessary to overcome the
back EMF of the fan. For example, one fan measured
started at 3.5V but operated until its terminal voltage fell
below 2.1V. Therefore, users must ensure start-up in the
fan before programming the fan voltage to a value lower
than the starting voltage. Monitoring the fan’s DC current
for a stalled condition does not work due to the fan’s
resistive nature. Fans can sink load current even though
they are not rotating. Other approaches include detecting
absence of the fan’s commutation ripple current and
tachometers. In general, these approaches are more com-
plex, require more circuitry, add cost and have to be
customized for the specific fan used.
The LTC1695 contains a programmable boost start timer
offering three flexible solutions to the user:
1.) Enable the boost start timer bit (D6 in the DAC com-
mand code). Each time a new output voltage is pro-
grammed, the timer forces V
OUT
to full scale (4.922V
nominal with V
CC
= 5V) for 250ms before assuming the
programmed output voltage value. This ensures fan start
up even if the programmed output voltage is below the
fan’s start threshold.
2.) Users may also choose to use a software timer routine
inside the host controller to power the DC fan, at full scale,
for a programmed time period before programming V
OUT
to a lower desired DAC output voltage code.
3.) Users may choose a tachometer fan that feedbacks its
speed to the SMBus host. If fan stall conditions are
detected, the SMBus host re-programs the LTC1695.
Beyond a typical 125°C LTC1695 junction temperature,
the boost start timer (if activated) maintains V
OUT
at full
APPLICATIONS INFORMATION
WUU
U
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC1695CS5#TRPBF

Mfr. #:
Buy LTC1695CS5#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Motor / Motion / Ignition Controllers & Drivers SMBus/I2C Fan Speed Cntr in SOT-23
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet