LTM8083 datasheet(13/24 Pages) AD

13 / 24 page

LTM8083

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Programming Current Limit Threshold

The ISP-ISN current is programmed by placing a cur-

rent sensing resistor, RCS, in series with the output. The

voltage drop across RCS is (Kelvin) sensed by the ISP and

ISN pins. The voltage on CTRL pin determines current

limit threshold voltage of ISP-ISN. The default setting

of the current limit threshold is full-scale 100mV. The

CTRL pin is pulled up to 2V reference voltage with a 100k

resistor internally. A resistor can be added between CTRL

and GND to form a voltage divider or an external voltage

below 1.3V can be applied to the CTRL pin to reduce the

current limit threshold of ISP-ISN across the output sense

resistor. When the CTRL pin voltage, VCTRL, is less than

1.15V, the current limit threshold is:

ICL=

VCTRL –250mV

10 • RCS

When VCTRL is between 1.15V and 1.35V, the current limit

threshold varies with VCTRL, but departs from the equa-

tion above by an increasing amount as VCTRL increases.

Ultimately, when VCTRL > 1.35V the output current thresh-

old is 100mV and no longer varies. The typical V(ISP-ISN)

threshold vs VCTRL is listed in Table 2.

Table 2. V(ISP-ISN) Threshold vs VCTRL

VCTRL (V)

V(ISP-ISN) (mV)

1.15

1.20

94.5

1.25

1.30

99.5

1.35

100

When VCTRL is higher than 1.35V, the output current limit

threshold is:

ICL=

100mV

RCS

The CTRL pin can also be used in conjunction with a

thermistor to provide overtemperature protection for the

output load, or with a resistor divider to VIN to reduce

output power and switching current when VIN is low.

The presence of a time varying differential voltage ripple

signal across ISP and ISN at the switching frequency is

expected. The amplitude of this signal is increased by

higher load current, lower switching frequency, or smaller

value output filter capacitor. Some level of ripple signal

is acceptable, and the compensation capacitor on the VC

pin filters the signal so the average difference between

ISP and ISN is regulated to the user-programmed value.

The ripple voltage amplitude (peak-to-peak) in excess

of 20mV should not cause misoperation, but may lead

to noticeable offset between the average value and the

user-programmed value.

Monitoring Sensed Current

The ISMON pin provides a linear indication of the cur-

rent flowing through the sensing resistor on the output.

It outputs a buffered and amplified monitor of the voltage

difference between ISP and ISN pins. The equation for

VISMON is:

VISMON =10 • V(ISP-ISN) + 250mV

Soft-Start

The SS pin can be used to program soft-start by connect-

ing an external capacitor CSS from the SS pin to ground.

A 10nF capacitor has been integrated inside the module.

The internal 32µA pull-up current charges up the capaci-

tor, creating a voltage ramp on the SS pin. As the SS pin

voltage rises linearly from 0.25V to 1V (and beyond), the

output voltage rises smoothly and transitions into output

voltage regulation. The soft-start range is defined to be

the voltage range from 0V to the FB voltage, which is

1V when output is regulated. The soft-start time can be

approximately calculated as:

tSS = 1V •

CSS + 10nF

32µA

Make sure the CSS is at least five to ten times larger than

the optional compensation capacitor on the VC pin.

Loop Compensation

The LTM8083 has already been internally compensated

and should be stable for all output voltages and capac-

itor combinations including all ceramic capacitor appli-

cations. If more switching noise attenuation is required,

an optional capacitor can be added on VC pin to ground

outside of the module. Furthermore, to further increase

LTM8083 데이터시트(PDF) 13 Page - Analog Devices

LTM8083 데이터시트(HTML) 13 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

데이터시트 다운로드

링크 URL