LTM4609

9

4609fa

OPERATION

Power Module Description

The LTM4609 is a non-isolated buck-boost DC/DC power

supply. It can deliver a wide range output voltage from

0.8V to 34V over a wide input range from 4.5V to 36V,

by only adding the sensing resistor, inductor and some

external input and output capacitors. It provides precisely

regulated output voltage programmable via one external

resistor. The typical application schematic is shown in

Figure 18.

The LTM4609 has an integrated current mode buck-boost

and integrated Schottky diodes. With current mode control

and internal feedback loop compensation, the LTM4609

module has sufficient stability margins and good transient

performance under a wide range of operating conditions

and with a wide range of output capacitors. The frequency

of LTM4609 can be operated from 200kHz to 400kHz by

setting the voltage on the PLLFLTR pin. Alternatively, its

frequency can be synchronized by the input clock signal

from the PLLIN pin. The typical switching frequency is

400kHz.

be enabled at light loads in the LTM4609 to improve its

efficiency, while the forced continuous mode and discon-

tinuous mode operations are used for constant frequency

applications. Foldback current limiting is activated in an

and undervoltage comparators pull the open-drain PGOOD

output low if the output feedback voltage exits the ±7.5%

window around the regulation point. Pulling the RUN pin

below 1.6V forces the controller into its shutdown state.

an efficiency improvement will occur due to the reduced

power loss in the internal linear regulator. This is especially

true at the higher input voltage range.

Burst Mode is a registered trademark of Linear Technology Corporation.

APPLICATIONS INFORMATION

The typical LTM4609 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 3 for specific external capacitor

requirements for a particular application.

Output Voltage Programming

The PWM controller has an internal 0.8V±1% reference

voltage. As shown in the Block Diagram, a 100k 0.5%

SGND pin programs the output voltage:

0.8V

1.5V

2.5V

3.3V

5V

6V

8V

9V

Open

115k

47.5k

32.4k

19.1k

15.4k

11k

9.76k

10V

12V

15V

16V

20V

24V

30V

34V

8.66k

7.15k

5.62k

5.23k

4.12k

3.4k

2.74k

2.37k

Operation Frequency Selection

The LTM4609 uses current mode control architecture at

constant switching frequency, which is determined by the

internal oscillator’s capacitor. This internal capacitor is

charged by a fixed current plus an additional current that

is proportional to the voltage applied to the PLLFLTR pin.

The PLLFLTR pin can be grounded to lower the frequency

to 200kHz or tied to 2.4V to yield approximately 400kHz.

When PLLIN is left open, the PLLFLTR pin goes low, forc-

ing the oscillator to its minimum frequency.

A graph for the voltage applied to the PLLFLTR pin vs

frequency is given in Figure 2. As the operating frequency

increases, the gate charge losses will be higher, thus the

efficiency is low. The maximum switching frequency is

approximately 400kHz.

FREQUENCY SYNCHRONIZATION

The LTM4609 can also be synchronized to an external

source via the PLLIN pin instead of adjusting the voltage on

the PLLFLTR pin directly. The power module has a phase-