LTM8049 datasheet(9/22 Pages) AD

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LTM4693

Rev. 0

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APPLICATIONS INFORMATION

The front page shows a typical LTM4693 application

circuit. This Applications Information section serves as

a guideline of selecting external components for typi-

cal applications. The examples and equations in this

section assume continuous conduction mode unless

otherwise specified.

VIN UVLO THRESHOLD

The VIN threshold is internally set to a typical value of

1.7V for turn-on and 1.6V for turn-off when RUN/UVLO

is connected to VIN. The VIN UVLO can be adjusted to a

higher threshold voltage with a resistor network on RUN/

UVLO according to Equation 4 and Equation 5.

VTURN(ON) =1.2V •(1+R1/R2)

(4)

The accurate RUN/UVLO pin threshold has 100mV of hys-

teresis provided internally.

VTURN(OFF) =1.1V •(1+R1/R2)

(5)

SETTING THE SWITCHING FREQUENCY

The operating frequency of the LTM4693 is optimized

to achieve the compact package size and the minimum

output ripple voltage while keeping high efficiency. The

default operating frequency is internally set to 1MHz by

an internal resistor. In most applications, no additional

frequency adjusting is required.

If any operating frequency higher than 1MHz is required

by application, the operating frequency can be adjusted

by adding a resistor RT between FREQ pin and GND. The

RT resistor required to set a specific switching frequency

can be calculated with Equation 7.

RT(kΩ)=

110

fSW(MHz)−1

(7)

The programmable operating frequency range is from

1MHz to 4MHz. The typical value of RT and the switching

frequency is shown as Table 1.

Table 1. RT Value for Common Switching Frequencies

fSW

1.0MHz

OPEN

2.0MHz

110kΩ

3.0MHz

55kΩ

4.0MHz

36.5kΩ

The LTM4693 can be synchronized to an external clock

applied to the MODE/SYNC pin. The frequency of the

external clock must be higher than the internal oscillator

frequency as set by the FREQ pin. In order to accom-

modate the ±20% possible variation in the oscillator fre-

quency, the RT resistor should be chosen to set the inter-

nal oscillator frequency between 25% to 50% below the

synchronization frequency. For example, to synchronize

to an external 2.5MHz clock, RT should be selected to set

the internal oscillator at 1.9MHz or lower.

OUTPUT VOLTAGE PROGRAMMING

The PWM controller has an internal 1V reference voltage.

As shown in the Block Diagram, a 60.4k internal feedback

resistor connects from VFB to VOUT. Adding a resistor

RFB from FB pin to GND pin programs the output voltage

(Equation 6).

VOUT =1.0V •

60.4k

+RFB

RFB

(6)

LTM4693

VIN

RUN/UVLO

4693 F01

Figure 1. Circuit to Set VIN UVLO

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