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LTM4646

4646f

For more information www.linear.com/LTM4646

The typical LTM4646 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 5 for specific external capacitor

requirements for particular applications.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step-

down ratio that can be achieved for a given input voltage.

Each output of the LTM4646 is capable of a wide duty

cycle that is limited by the minimum on-time tON(MIN) of

30ns defined as tON(MIN) < D/fSW for narrow duty cycle,

where D is duty cycle (VOUT/VIN) and fSW is the switch-

ing frequency. The minimum off-time of 90ns tOFF(MIN)

< 1 – D/fSW is required for higher duty cycles. See Note 8

in the Electrical Characteristics section for output current

guideline.

Output Voltage Programming

The PWM controller has an internal 0.6V reference voltage.

As shown in Figure 1, a 60.4k internal feedback resistor

connects between the VOUTS1 to VFB1 and VOUTS2 to VFB2.

It is very important that these pins be connected to their

respective outputs for proper feedback regulation. Each

channel has a 49.9Ω resistor connected from VOUTS1 and

VOUTS2 to VOUT1 and VOUT2, respectively. This is used to

protect the output if VOUTSn is open or left unconnected.

The VOUT1 output voltage will default to 0.6V with no feed-

back resistor on VFB1. Adding a resistor RFB1 from VFB1

pin to VOUTS1– programs the output voltage:

VOUT1 0.6V •

60.4k RFB1

RFB1

The VOUT2 output voltage will default to 0.3V with no feed-

back resistor on VFB2. Adding a resistor RFB2 from VFB2

pin to VOUTS2–, and the RFB3 resistor equal to (60.4k//

RFB2) from VFB2 to SGND programs the output voltage:

VOUT2 0.6V •

60.4k RFB2

RFB2

RFB3 60.4k ||RFB2

60.4k • RFB2

60.4k + RFB2

APPLICATIONS INFORMATION

The Thevenin equivalent of the VOUT2 equation would

be the 0.6V with a series resistance of (60.4k || RFB2),

thus RFB3 connected to the series resistance would be

(60.4k||RFB2) to equal the 0.3V reference.

Table 1. VFB1, VFB2, Resistor Table vs Various OutputVoltages

VOUT1 0.6V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V 5.0V

RFB1 Open 90.9k 60.4k 40.2k 30.1k 19.1k 13.3k 8.25k

VOUT2 0.3V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V 5.0V

RFB2 Open 90.9k 60.4k 40.2k 30.1k 19.1k 13.3k 8.25k

RFB3 Open 36.5k 30.1k 24.3k 20k 14.7k 11k 7.32k

Figure 2 shows the LTM4646 used in a 2-phase single

output: Tie the VFB1 pin to INTVCC, which will disable

channel 1’s error amplifier and internally connect COMP2

to COMP1. Tie any of the compensation components to

the COMP2 pin. The COMP1 pin can be either left open

or shorted to COMP2 externally as shown. The, VOUTS1,

TRACK/SS1 and PGOOD1 pins become non-operable and

can be left open. To make a single-output converter of

three or more phases, additional LTM4646 micromod-

ules can be used. The first module should be tied the

same way as the Figure 2. If only one more channel of

an additional LTM4646 is needed, use channel 2 for the

additional phase:

• Tie the COMP2 pin to the COMP2 pin of the first module.

• Tie the RUN2 pin to the RUN pins of the first module.

Use 1/2 the value for RFB2 and RFB3.

• Tie VOUTS2 of the additional channel to VOUTS2 of the

first module then to remote sense point.

• Tie the VFB2 pin to the VFB2 pin of the first module.

• Tie the VOUTS2– pin to the VOUTS2– pin of the

firstmodule.

• Tie the TRACK/SS2 pin to the TRACK/SS2 pin of the

first module.

If both channels are needed for four phases, the addi-

tional LTM4646 module should be tied the same way

as the first as shown in Figure 2 to disable the second

channel1’s EA:

• Tie the VFB1 pin to the module’s own INTVCC.

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