Data Sheet

LTM4640

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Rev. 0

15 of 38

APPLICATIONS INFORMATION

The typical LTM4640 application circuit is shown in Figure 37. The external component selection is primarily

determined by the input voltage, output voltage, load transient, and maximum load current. See Table 12 for

specific external capacitor requirements for a particular application.

V

IN to VOUT Step-Down Ratios

There are restrictions on the maximum VIN and VOUT step-down ratios that can be achieved for a given input voltage

because of the minimum off-time and minimum on-time limits of the regulator. The minimum off-time limit

imposes a maximum duty cycle which is calculated with Equation 1.

������������������������ = 1 − (������������������������(������������������) × ������������������)

(1)

where tOFF(MIN)is the minimum off-time, typically 50ns for LTM4640, and fSW (Hz) is the switching frequency.

Conversely, the minimum on-time limit imposes a minimum duty cycle of the converter which is calculated

with Equation 2.

������������������������ = ������������������(������������������) × ������������������

(2)

where tON(MIN)is the minimum on-time, typically 25ns for LTM4640. In the rare cases where the minimum duty cycle

is surpassed, the output voltage remains in regulation, but the switching frequency decreases from its

programmed value. Note that additional thermal derating may be applied. See the Thermal Considerations and

Output Current Derating section.

Output Voltage Programming

The pulse-width modulation (PWM) controller has an internal 0.6V reference voltage. As shown in Figure 17

(Simplified Block Diagram), a 10kΩ internal feedback resistor connects the VOSNS+ and the FB pins. Adding a resistor,

RFB, from FB pin to VOSNS– pin programs the output voltage given by Equation 3.

������������������ =

0.6������

������������������������ − 0.6������

× 10������Ω

(3)

Table 7 summarizes the RFB values required for some of the typical output voltage applications.

Table 7. RFB Resistor Table vs. Various Output Voltages

VOUT (V)

0.6

1.0

1.2

1.5

1.8

2.5

3.3

RFB (kΩ)

OPEN

15

10

6.65

4.99

3.16

2.21

For parallel operation of multiple channels, the same feedback setting resistor is used for the parallel design. This is

done by connecting the VOSNS+ to the output, as shown in Figure 18, thus connecting one of the internal 10kΩ

resistors to the output. All the VFB pins connect with one programming resistor, as shown in Figure 18. See Figure 39

for an example of a parallel operation.