LTC1695CS5 datasheet(13/20 Pages) LINER

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LTC1695

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.

≥1µFgeneralpurpose,lowtomediumESR(0.1Ωto5Ω)

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 4.7

µF, 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 (<0.1

Ω) 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

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 10

µF or greater tantalum capacitor to

bypass VCC. Users must account for the variation in the

DAC’s output absolute accuracy as VCCvaries. 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.

Figure 3. Ladder DAC

64 RESISTOR

VOLTAGE TABS

720

SWITCHES

VCC

GND

REFERENCE

OP AMP

“000000” = 0V

“111111” = 0.984 • VCC/2

SMBus

COMMAND

D5 to D0

VCC/2

1695 • F03

APPLICATIONS INFORMATION

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