LT1963 datasheet(19/26 Pages) LINER

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LT3088

3088fb

For more information www.linear.com/LT3088

The second technique for reducing power dissipation,

shown in Figure 15, uses a resistor in parallel with the

LT3088. This resistor provides a parallel path for current

flow, reducing the current flowing through the LT3088.

This technique works well if input voltage is reasonably

constant and output load current changes are small. This

technique also increases the maximum available output

current at the expense of minimum load requirements.

As an example, assume: VIN = 5V, VIN(MAX) = 5.5V, VOUT

= 3.3V, VOUT(MIN) = 3.2V, IOUT(MAX) = 0.8A and IOUT(MIN)

= 0.3A. Also, assuming that RP carries no more than 90%

of IOUT(MIN) = 270mA.

Calculating RP yields:

RP =

5.5V – 3.2V

0.27A

= 8.52Ω

(5% Standard value = 9.1Ω)

The maximum total power dissipation is:

(5.5V – 3.2V) • 0.8A = 1.84W

However, the LT3088 supplies only:

0.8A –

5.5V – 3.2V

9.1Ω

= 0.55A

Therefore, the LT3088’s power dissipation is only:

PDISS = (5.5V – 3.2V) • 0.55A = 1.26W

RP dissipates 0.58W of power. As with the first technique,

choose appropriate wattage resistors to handle and dis-

sipate the power properly. With this configuration, the

LT3088 supplies only 0.55A. Therefore, load current can

increase by 0.25A to a total output current of 1.05A while

keeping the LT3088 in its normal operating range.

High Temperature Operation

Care must be taken when designing the LT3088H/

LT3088MP applications to operate at high ambient tem-

peratures. The LT3088H/LT3088MP operates at high

temperatures, but erratic operation can occur due to un-

foreseenvariationsinexternalcomponents.Sometantalum

capacitorsareavailableforhightemperatureoperation,but

ESR is often several ohms; capacitor ESR above 0.5Ω is

unsuitable for use with the LT3088H/LT3088MP. Multiple

ceramiccapacitormanufacturersnowofferceramiccapaci-

tors that are rated to 150°C using an X8R dielectric. Check

each passive component for absolute value and voltage

ratings over the operating temperature range.

Leakages in capacitors or from solder flux left after insuf-

ficient board cleaning adversely affects low current nodes,

such as the SET pins. Consider junction temperature in-

crease due to power dissipation in both the junction and

nearby components to ensure maximum specifications

are not violated for the LT3088H/LT3088MP or external

components.

APPLICATIONS INFORMATION

3088 F15

SET

OUT

–

LT3088

50µA

RSET

VOUT

VIN

Figure 15. Reducing Power Dissipation Using a Parallel Resistor

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