LTM8080 datasheet(16/32 Pages) AD

16 / 32 page

LTM4703

Rev. 0

For more information www.analog.com

operation. If the SVIN and PVIN are powered from different

sources, keep SVIN above 3V to ensure proper operation.

The voltage transients or ripple valleys that cause the SVIN

to fall below 3V may turn off the LTM4703.

SET Pin (Bypass) Capacitance: Noise, Transient

Response, and Soft-Start

In addition to reducing output noise, using a SET pin

bypass capacitor reduces sensitivity to any parasitic cou-

pling of voltage spikes onto the SET pin. Note that any

bypass capacitor leakage deteriorates the LTM4703 DC

regulation. Capacitor leakage of even 100nA is a 0.1% DC

error. Therefore, it is recommended to use a good-quality,

low-leakage ceramic capacitor.

Also, using a SET pin bypass capacitor soft-starts the

output and limits inrush current. Soft-starting the output

prevents a current surge in the input supply. The SET pin

capacitor and resistor values set the ramp-up time of the

reference voltage, and the output voltage tracks this volt-

age. The SET pin resistor size is determined by the appli-

cation’s desired output voltage; however, the capacitor

size may be selected to achieve the desired ramp-up time.

It is important to remember that the size of the SET pin

capacitor also plays a role in noise performance, which

is typically the more important factor in determining the

size of this capacitor.

Without fast start-up enabled, the RC time constant,

formed by the SET pin resistor and capacitor, controls

soft-start time. Connect the PGSET pin to 0.5V to disable

fast start-up. The ramp-up rate from 0% to 90% of nom-

inal VOUT is given by Equation 3.

tSTART_NO_FAST_START-UP = 2.3•RSET •CSET

(3)

With fast start-up enabled, the start-up time can be sig-

nificantly reduced with the ramp-up time from 0% to 90%

of nominal VOUT as shown by Equation 4.

tSTART_FAST_START-UP =

100µA •RSET •CSET

2.7mA

(4)

In most applications, fast start-up is enabled. In this case,

a minimum SET capacitor size of 1μF is strongly rec-

ommended to prevent reference voltage overcharge and

ensure good noise performance.

Soft-Start and Power Sequencing

As discussed in the SET Pin (Bypass) Capacitance: Noise,

Transient Response, and Soft-Start section, soft-start is

achieved through the controlled ramp-up time of the SET

pin voltage. The soft-start is guaranteed when all PVIN and

SVIN pins are connected.

When PVIN and SVIN are powered by independent

supplies, power sequencing must be considered to guar-

antee a soft-start. The SET pin voltage should start at 0V

when PVIN is applied. To guarantee a soft-start, do not

power PVIN last when sequencing PVIN, SVIN, and RUN.

An example of a specific case to avoid is having SVIN and

RUN powered up before PVIN. In this instance, the SET

pin voltage rises to a voltage greater than 0V when PVIN

is applied, and the LTM4703 does not soft-start correctly.

Fast Start-Up

For ultralow noise applications that require low 1/f noise

(i.e., at frequencies below 100Hz), a larger value SET pin

capacitor is required, up to 22μF. A larger value capacitor

can be used, but care should be taken regarding leak-

age. While normally larger capacitors would significantly

increase the regulator’s start-up time, the LTM4703 incor-

porates fast start-up circuitry, which increases the SET pin

current to about 2.7mA during start-up.

Upon start-up, the 2.7mA current source remains

engaged while PGSET is below the power good threshold

of 462.5mV, unless the regulator is in thermal shutdown,

SVIN is too low, or INTVCC has fallen too low.

The fast start-up circuit is disabled permanently once

PGSET rises above the power good threshold, until either

the part is powered down, or the part is placed into shut

down by pulling the RUN pin to GND.

There is one more condition under which the 2.7mA

current source is disabled during start-up. The purpose

of this is to prevent overcharging VSET. Since the part

assumes that the PGSET pin is an accurately indicates

the voltage on the SET pin, it assumes that VOSNS follows

VSET closely. However, this may not always be the case.

For example, if the output capacitance is very large, or if,

for some reason, the output is shorted to GND. Therefore,

fast charge is also disabled whenever the COMPa pin has

APPLICATIONS INFORMATION

LTM8080 Datasheet(PDF) 16 Page - Analog Devices

LTM8080 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL