LTC4090EDJC(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4090EDJC
(Rev.:RevA)

USB Power Manager with 2A High Voltage Bat-Track Buck Regulator

Linear Technology 

LTC4090/LTC4090-5

APPLICATIONS INFORMATION

Boost Pin Considerations

Capacitor C2 (see Block Diagram) and an internal diode

are used to generate a boost voltage that is higher than

the input voltage. In most cases, a 0.47μF capacitor will

work well. The BOOST pin must be at least 2.3V above

the SW pin for proper operation.

High Voltage Regulator Soft-Start

The HVEN pin can be used to soft-start the high voltage

regulator of the LTC4090/LTC4090-5, reducing maximum

input current during start-up. The HVEN pin is driven

through an external RC filter to create a voltage ramp at

this pin. Figure 7 shows the start-up and shutdown wave-

forms with the soft-start circuit. By choosing a large RC

time constant, the peak start-up current can be reduced

to the current that is required to regulate the output, with

no overshoot. Choose the value of the resistor so that it

can supply 20μA when the HVEN pin reaches 2.3V.

RUN

15k

0.22μF

HVEN

GND

IL

1A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

2ms/DIV

4090 F07

Figure 7. To Soft-Start the High Voltage Regulator, Add a Resistor

and Capacitor to the HVEN Pin

Synchronization and Mode

The SYNC pin allows the high voltage regulator to be

synchronized to an external clock.

Synchronizing the LTC4090/LTC4090-5 internal oscilla-

tor to an external frequency can be done by connecting a

square wave (with 20% to 80% duty cycle) to the SYNC

pin. The square wave amplitude should be such that the

valleys are below 0.3V and the peaks are above 0.8V (up

to 6V). The high voltage regulator may be synchronized

over a 300kHz to 2MHz range. The RT resistor should be

chosen such that the LTC4090/LTC4090-5 oscillate 25%

22

lower than the external synchronization frequency to ensure

adequate slope compensation. While synchronized, the

high voltage regulator will turn on the power switch on

positive going edges of the clock. When the power good

(PG) output is low, such as during start-up, short-circuit,

and overload conditions, the LTC4090/LTC4090-5 will dis-

able the synchronization feature. The SYNC pin should be

grounded when synchronization is not required.

Alternate NTC Thermistors and Biasing

The LTC4090/LTC4090-5 provide temperature qualified

charging if a grounded thermistor and a bias resistor are

connected to NTC (see Figure 8). By using a bias resistor

whose value is equal to the room temperature resistance

of the thermistor (R25C) the upper and lower temperatures

are pre-programmed to approximately 50°C and 0°C,

respectively (assuming a Vishay “Curve 2” thermistor).

The upper and lower temperature thresholds can be ad-

justed by either a modification of the bias resistor value

or by adding a second adjustment resistor to the circuit.

If only the bias resistor is adjusted, then either the upper

or the lower threshold can be modified but not both. The

other trip point will be determined by the characteristics

of the thermistor. Using the bias resistor in addition to an

adjustment resistor, both the upper and the lower tempera-

ture trip points can be independently programmed with

the constraint that the difference between the upper and

lower temperature thresholds cannot decrease. Examples

of each technique are given below.

NTC thermistors have temperature characteristics which

are indicated on resistance-temperature conversion tables.

The Vishay-Dale thermistor NTHS0603N02N1002J, used

in the following examples, has a nominal value of 10k

and follows the Vishay “Curve 2” resistance-temperature

characteristic. The LTC4090/LTC4090-5’s trip points are

designed to work with thermistors whose resistance-tem-

perature characteristics follow Vishay Dale’s “R-T Curve 2.”

The Vishay NTHS0603N02N1002J is an example of such

a thermistor. However, Vishay Dale has many thermistor

products that follow the “R-T Curve 2” characteristic in a

variety of sizes. Furthermore, any thermistor whose ratio

of RCOLD to RHOT is about 7.0 will also work (Vishay Dale

R-T Curve 2 shows a ratio of 2.815/0.409 = 6.89).

4090fa

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