LTC3129 datasheet(23/34 Pages) AD

23 / 34 page

LTC3114-1

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

applications are generally similar except that voltage ripple

is generally not a concern. Some capacitors exhibit a high

DC leakage current which may preclude their consideration

for applications that require a very low quiescent current in

Burst Mode operation.

Ceramic capacitors are often utilized in switching con-

verter applications due to their small size, low ESR and

low leakage currents. However, many ceramic capacitors

intended for power applications experience a significant

loss in capacitance from their rated value as the DC bias

voltage on the capacitor increases. It is not uncommon for

a small surface mount capacitor to lose more than 50%

of its rated capacitance when operated near its maximum

rated voltage. This effect is generally reduced as the case

size is increased for the same nominal value capacitor. As

a result, it is often necessary to use a larger value capaci-

tance or a higher voltage rated capacitor than would ordi-

narily be required to actually realize the intended capaci-

tance at the operating voltage of the application. X5R and

X7R dielectric types are recommended as they exhibit

the best performance over the wide operating range and

temperature of the LTC3114-1. To verify that the intended

capacitance is achieved in the application circuit, be sure

to consult the capacitor vendor’s curve of capacitance

versus DC bias voltage.

Programming Custom VIN Turn-On and Turn-Off

Thresholds

With the addition of an external resistor divider connected

to the input voltage as shown in Figure 3, the RUN pin

can be used to program the input voltage at which the

LTC3114-1 is enabled and disabled.

For a rising input voltage, the LTC3114-1 is enabled when

VIN reaches the threshold given by the following equation,

where R1 and R2 are the values of the resistor divider

resistors specified in kΩ:

VTH(RISING) = 1.2

+R2

⎛

⎝⎜

⎞

⎠⎟

Volts

Once the IC is enabled, it will remain so until the input

voltage drops below the comparator threshold by the

hysteresis voltage of approximately 100mV, measured

on the RUN pin. Therefore, the amount of hysteresis is

approximately 8.33% of the programmed turn-on thresh-

old level given in the previous equation.

Bootstrapping the LDO Regulator

The high and low side gate drivers are powered through

the PLDO rail, which is generated from the input voltage,

VIN, through an internal linear regulator. In some applica-

tions, especially at high input voltages, the power dissipa-

tion in the linear regulator can become a major contributor

to thermal heating of the IC. The Typical Performance

Characteristics section of this data sheet provides data on

the LDO/PLDO current and resulting power loss versus

VIN and VOUT. A significant performance advantage can

be attained in applications where converter output voltage

(VOUT) is programmed to 5V, if VOUT is used to power

the LDO/PLDO rails. Powering the LDO/PLDO rails in this

manner is referred to as bootstrapping. This can be done

by connecting a Schottky diode from VOUT to LDO/PLDO

as shown in Figure 5. With the bootstrap diode installed,

the gate driver currents are supplied by the buck-boost

converter at high efficiency rather than through the inter-

nal linear regulator. The internal linear regulator contains

reverse blocking circuitry that allows LDO/PLDO pins to

be driven slightly above their nominal regulation level with

only a very slight amount of reverse current. Please note

that the bootstrapping supply (either VOUT or a separate

regulator) must be limited to less than 5.7V.

VOUT

4.7µF

31141 F05

PVOUT

LTC3114-1

LDO

PLDO

Figure 5. Bootstrapping PLDO and LDO

Average Output Current Limit Programming

The LTC3114-1 includes an average output current pro-

gramming feature that transforms the LTC3114-1 into

a wide voltage compliance range, high efficiency, con-

stant current source. A resistor from PROG to ground

programs the desired level of average output current up

to 1A. Potential uses include high brightness LED driving

and constant current battery or capacitor charging.

LTC3129 Datasheet(PDF) 23 Page - Analog Devices

LTC3129 Datasheet(HTML) 23 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 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL