LT3045 datasheet(39/45 Pages) AD

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Data Sheet

LT3097

APPLICATIONS INFORMATION

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Rev. 0 | 39 of 45

threshold for the negative regulator as illustrated in the Figure 131.

The ENN/UVN pin current (IENN/UVN) at the threshold from Table 1

must be considered when calculating the resistor-divider network as

follows:

VINN UVLO =−1.26 V× 1+RENN2RENN1

−IENN/UVN×RENN2

(3)

where:

RENN1 and RENN2 are the resistors from the ENN/UVN pin to GND

and the ENN/UVN pin to INN, respectively.

IENN/UVN can be ignored if RENN1 is less than 100 kΩ. If unused,

connect the ENP/UVP pin to INP.

Since the ENN/UVN pin is bidirectional, it can also be pulled more

than 1.26V to turn on the negative regulator of the LT3097. In

bipolar supply applications, the positive ENN/UVN threshold can

be used to sequence the turn-on of the negative regulator of the

LT3097 after the positive regulator of the LT3097 has turned on. If

unused, connect the ENN/UVN pin to INN.

POSITIVE REGULATOR PROGRAMMABLE

POWER GOOD

As shown in the Figure 122 and discussed in the Fast Startup sec-

tion, the positive side power-good threshold is user-programmable

using the ratio of two external resistors, RPGP1 and RPGP2:

VOUTPPG_THRESHOLD =0.3V× 1+RPGP2RPGP1

+IPGFBP×RPGP2

(4)

If the PGFBP pin increases to more than 300 mV, the open-collec-

tor PGP pin deasserts and becomes high impedance. The power-

good comparator has 7 mV hysteresis and 5 μs of deglitching.

The IPGFBP from Table 1 must be considered when determining the

resistor-divider network. The IPGFBP can be ignored if RPGP1 is less

than 30 kΩ. If the power-good functionality is not used, float the

PGP pin. Note that programmable power good and fast start-up

capabilities are disabled for positive output voltages less than 300

mV.

The power-good functionality is disabled in shutdown, i.e. when

ENP/UVP is set to 0V. If power-good functionality is desired in

shutdown, connect the power-good resistor (i.e. RPGP in the Figure

122) between the PGP pin and either the ENP/UVP pin or OUTP

pin.

NEGATIVE REGULATOR PROGRAMMABLE

POWER GOOD

As shown in the Figure 123 and discussed in the Fast Startup sec-

tion, the negative side power-good threshold is user-programmable

using the ratio of two external resistors, RPGN1 and RPGN2:

VOUTNPG_THRESHOLD =−0.3V× 1+RPGN2RPGN1

−IPGFBN×RPGN2

(5)

If the PGFBN pin decreases to less than −300 mV, the open-col-

lector PGN pin deasserts and becomes high impedance. The pow-

er-good comparator has 7 mV hysteresis and 5 μs of deglitching.

The IPGFBN from Table 1 must be considered when determining the

resistor-divider network. The IPGFBN can be ignored if RPGP1 is less

than 30 kΩ. If the power-good functionality is not used, float the

PGN pin. Note that programmable power good and fast start-up

capabilities are disabled for negative output voltages between 0 V

and −300 mV.

Take care when laying out traces for PGN and PGFBN on a PCB.

If the PGN and PGFBN pins are run close to each other for a

distance (typically greater than two inches), stray capacitance from

trace-to-trace couples the PGN signal into the high-impedance

PGFBN signal. Since PGN is out of phase relative to PGFBN, this

results in oscillation. To avoid this, minimize the distance the two

traces run close to each other; lowering the impedance seen at the

PGFBN pin by using lower value resistors for the PGFBN divider

also helps.

POSITIVE REGULATOR EXTERNALLY

PROGRAMMABLE CURRENT LIMIT

The current-limit threshold of the ILIMP pin is 300 mV. Connecting

a resistor from ILIMP to GND sets the maximum current flowing out

of the ILIMP pin, which, in turn, programs the current limit of the

positive regulator of the LT3097. With a 150 mA × kΩ programming

scale factor, calculate the current limit as follows:

Positive Side Current Limit=150 mA×kΩ

RILIMP (6)

For example, a 1 kΩ resistor programs the current limit to 150 mA,

and a 2 kΩ resistor programs the current limit to 75 mA. For good

accuracy, Kelvin connect this resistor to the GND pin (pin 19) of the

LT3097.

When the INP-to-OUTP differential is greater than 12 V, the fold-

back circuitry of the positive regulator of the LT3097 decreases

the internal current limit. As a result, the internal current limit can

override the externally programmed current-limit level to keep the

LT3097 within its safe-operating area (SOA). See Figure 56.

As shown in the Figure 122, the ILIMP pin sources current propor-

tional (1:500) to the output current; therefore, it also serves as a

current monitoring pin with a 0 V to 300 mV range. If external

current limit or current monitoring is not used, connect ILIMP to

GND.

NEGATIVE REGULATOR EXTERNALLY

PROGRAMMABLE CURRENT LIMIT

The ILIMN pin internally regulates to −300 mV. Connecting a resis-

tor from GND to ILIMN sets the current flowing into the ILIMN pin,

which, in turn, programs the current limit of the negative regulator of

the LT3097. With a 3.75 A × kΩ programming scale factor, calculate

the current limit as follows:

LT3045 Datasheet(PDF) 39 Page - Analog Devices

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