ADP151 datasheet(31/47 Pages) AD

31 / 47 page

Data Sheet

LT3046

analog.com

Rev 0

31 of 47

��(��) = 1.27�� × (1 +

��2

��1

) + ��/�� × ��2

where:

REN1 and REN2 are the resistors from the EN/UV pin to GND and the EN/UV pin to IN, respectively.

IEN/UV can be ignored if REN1 is less than 100kΩ. If unused, connect the EN/UV pin to IN.

Programmable Power Good

As illustrated in Figure 66, the power-good threshold is user programmable using the ratio of two external resistors,

RPG2 and RPG1:

��(��_��) = 0.298�� × (1 +

��2

��1

) + �� × ��2

If the PGFB pin increases to more than 298mV, the open-drain PG pin deasserts and becomes high impedance. The

power-good comparator has 7mV hysteresis and 20μs of deglitching. The IPGFB from Table 1 must be considered

when determining the resistor-divider network. The IPGFB can be ignored if RPG1 is less than 30kΩ. If the power-good

functionality is not used, float the PG pin. Note that programmable power good and fast start-up capabilities are

disabled for output voltages less than 298mV. Connect FGFB to IN if the power good and fast start-up functions are

not needed.

Externally Programmable Current Limit

The current-limit threshold of the ILIM pin is 290mV. Connecting a resistor from ILIM to GND sets the maximum

current flowing out of the ILIM pin, which, in turn, programs the current limit of the LT3046. With a 150mA × kΩ

programming scale factor, calculate the current limit as follows:

�� =

150�� × ��Ω

��

For example, a 750Ω resistor programs the current limit to 200mA and a 1.5kΩ resistor programs the current limit to

100mA. For accuracy, Kelvin connect this resistor to the GND pin of the LT3046.

When the IN-to-OUT differential is greater than 11V, the foldback circuitry of the LT3046 decreases the internal

current limit. As a result, the internal current limit can override the externally programmed current-limit level to

keep the LT3046 within its safe-operating area (SOA). See Figure 32.

As shown in Figure 66, the ILIM pin sources current proportional (1:520) to the output current; therefore, it also

serves as a current-monitoring pin with a 0V to 290mV range. If external current limit or current monitoring is not

used, connect ILIM to GND.

Output Overshoot Recovery

During a load-step change from full load to no load (or light load), the output voltage overshoots before the

regulator responds to turn the power transistor off. Given that there is no load (or a light load) present at the

output, it takes a long time to discharge the output capacitor.

As shown in Figure 66, the LT3046 incorporates overshoot recovery circuitry that turns on a current sink to

discharge the output capacitor in the event OUTS is higher than SET. This current is typically approximately 4mA.

No load recovery is disabled for input voltages less than 2.5V or output voltages less than 1.5V.

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