LM20145 datasheet(18/29 Pages) TI

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RA =

- 1

VTO

VIH_EN

x RB

VOUT1

LM20145

External

Power Supply

VOUT2

tSS =

0.8V x CSS

ISS

LM20145

SNVS530E – OCTOBER 2007 – REVISED MARCH 2013

www.ti.com

Table 3. Start Up Times for Different Soft-Start Capacitors

Start Up Time (ms)

CSS (nF)

none

100

120

If different start up times are needed the equation shown below can be used to calculate the start up time.

(12)

As shown above, the start up time is influenced by the value of the Soft-Start capacitor CSS(F) and the 5 µA Soft-

Start pin current ISS(A). that may be found in the electrical characteristics table.

While the Soft-Start capacitor can be sized to meet many start up requirements, there are limitations to its size.

The Soft-Start time can never be faster than 1ms due to the internal default 1 ms start up time. When the device

is enabled there is an approximate time interval of 50 µs when the Soft-Start capacitor will be discharged just

prior to the Soft-Start ramp. If the enable pin is rapidly pulsed or the Soft-Start capacitor is large there may not be

enough time for CSS to completely discharge resulting in start up times less than predicted. To aid in discharging

of Soft-Start capacitor during long disable periods an external 1 M

Ω resistor from SS/TRK to ground can be used

without greatly affecting the start-up time.

USING PRECISION ENABLE AND POWER GOOD

The precision enable (EN) and power good (PGOOD) pins of the LM20145 can be used to address many

sequencing requirements. The turn-on of the LM20145 can be controlled with the precision enable pin by using

two external resistors as shown in Figure 30.

Figure 30. Sequencing LM20145 with Precision Enable

The value for resistor RB can be selected by the user to control the current through the divider. Typically this

resistor will be selected to be between 10 k

Ω and 1 MΩ. Once the value for RB is chosen the resistor RA can be

solved using the equation below to set the desired turn-on voltage.

(13)

When designing for a specific turn-on threshold (VTO) the tolerance on the input supply, enable threshold

(VIH_EN), and external resistors needs to be considered to insure proper turn-on of the device.

The LM20145 features an open drain power good (PGOOD) pin to sequence external supplies or loads and to

provide fault detection. This pin requires an external resistor (RPG) to pull PGOOD high while when the output is

within the PGOOD tolerance window. Typical values for this resistor range from 10 k

Ω to 100 kΩ.

TRACKING AN EXTERNAL SUPPLY

By using a properly chosen resistor divider network connected to the SS/TRK pin, as shown in Figure 31, the

output of the LM20145 can be configured to track an external voltage source to obtain a simultaneous or

ratiometric start up.

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