LTM8083 datasheet(10/24 Pages) AD

10 / 24 page

LTM8083

Rev. 0

For more information www.analog.com

The front page shows a typical LTM8083 application

circuit. This Applications Information section serves as

a guideline of selecting external components for typical

applications. The examples and equations in this section

assume continuous conduction mode unless otherwise

specified.

Programming Output Voltage and Thresholds

The LTM8083 has a voltage feedback pin FB that can be

used to program a constant-voltage output. The output

voltage can be set by selecting the values of R5 and R6

(Figure 1) according to the following equation:

VOUT = 1.00V •

+ R6

APPLICATIONS INFORMATION

Switching Frequency Setting

The switching frequency of the LTM8083 can be set by the

internal oscillator. With the SYNC pin pulled to ground, the

switching frequency is set by a resistor from the RT pin

to ground. Table 1 shows RT resistor values for common

switching frequencies.

Table 1. Switching Frequency vs RT Value (1% Resistor)

fOSC (kHz)

RT (kΩ)

300

178

400

124

600

78.7

800

56.2

1000

40.2

1200

33.2

1400

26.1

1600

21.5

1800

17.4

2000

14.3

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome for

applications where electromagnetic interference (EMI) is a

concern. To improve the EMI performance, the LTM8083

implements a triangle spread spectrum frequency mod-

ulation scheme. With the SYNC pin tied to INTVCC, the

LTM8083 starts to spread its switching frequency 25%

above the internal oscillator frequency. Besides, a low-ESL

three-terminal capacitor is added on the input side as well

as output side to further improve the EMI performance.

Figure 2 and Figure 3 show the noise spectrum of the

front page application with spread spectrum enabled and

a small EMI filter at 12VIN, 12VOUT, and 1A load current.

The detailed schematic is shown in Figure 25.

Frequency Synchronization

The LTM8083 switching frequency can be synchronized

to an external clock using the SYNC pin. Driving the SYNC

with a 50% duty cycle waveform is always a good choice,

otherwise maintain the duty cycle between 10% and 90%.

Due to the use of a phase-locked loop (PLL) inside, there

is no restriction between the synchronization frequency

Figure 1. Feedback Resistor Connection

LTM8083

GND

8083 F05

VOUT

In addition, the FB pin also sets output overvoltage thresh-

old. Once the FB pin hits its overvoltage threshold 1.05V,

the LTM8083 stops switching by turning off all four power

switches for protection. The output overvoltage threshold

can be set as:

VOUT(OVP) = 1.05V •

+ R6

Switching Frequency Selection

The LTM8083 uses a constant frequency control scheme

between 300kHz and 2MHz. Selection of the switching

frequency is a trade-off between efficiency and capacitor

size to filter input and output voltage switching noise.

Low frequency operation improves efficiency by reducing

MOSFET switching loss but requires larger capacitor val-

ues. In a noise-sensitive system, the switching frequency

is usually selected to keep the switching noise out of a

sensitive frequency band. For most applications, 1MHz

switching frequency is recommended.

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