Datasheet LTM4644, LTM4644-1 (Analog Devices) - 10

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APPLICATIONS INFORMATION
The typical LTM4644 application circuit is shown in For parallel operation of N channels, use the following Figure 33. External component selection is primarily equation can be used to solve for RFB(BOT). Tie the VOUT determined by the input voltage, the output voltage and and the FB and COMP pins together for each paralleled the maximum load current. Refer to Table 7 for specific output with a single resistor to GND as determined by: external capacitor requirements for a particular application. ⎛60.4k ⎞ ⎜ ⎟
V
⎝ N
IN to VOUT Step-Down Ratios
R ⎠ FB(BOT) = ⎛ V ⎞ There are restrictions in the maximum V OUT IN and VOUT step- ⎜ −1⎟ down ratio that can be achieved for a given input voltage ⎝ 0.6 ⎠ due to the minimum off-time and minimum on-time limits
OUTPUT VOLTAGE PROGRAMMING (LTM4644-1)
of each regulator. The minimum off-time limit imposes a The PWM controller has an internal 0.6V reference voltage. maximum duty cycle which can be calculated as: Adding two resistors RFB(TOP) from VOUT to FB pin and DMAX = 1 – tOFF(MIN) • fSW RFB(BOT) from FB pin to GND programs the output voltage: where tOFF(MIN) is the minimum off-time, 70ns typical for R LTM4644, and f FB(TOP) SW is the switching frequency. Conversely RFB(BOT) = the minimum on-time limit imposes a minimum duty cycle VOUT

–1 of the converter which can be calculated as: 0.6 For parallel operation of N Channels, only one set of DMIN = tON(MIN) • fSW RFB(TOP) and RFB(BOT) is needed while tying the VOUT, FB where tON(MIN) is the minimum on-time, 40ns typical for and COMP pins from different channels together. See LTM4644. In the rare cases where the minimum duty Figure 1 for example. cycle is surpassed, the output voltage will still remain in regulation, but the switching frequency will decrease VOUT1 from its programmed value. Note that additional thermal RFB(TOP) derating may be applied. See the Thermal Considerations LTM4644-1 FB1 and Output Current Derating section in this data sheet. RFB(BOT) COMP1 VOUT2
Output Voltage Programming (LTM4644)
FB2 COMP2 The PWM controller has an internal 0.6V reference voltage. VOUT3 As shown in the Block Diagram, a 60.4k internal feedback FB3 COMP3 resistor connects each regulator channel from VOUT pin 4644 F01 to FB pin. Adding a resistor RFB(BOT) from FB pin to GND programs the output voltage:
Figure 1. LTM4644-1 Feedback Resistor for Paralleling Application
R 60.4k FB(BOT) =
LTM4644 LTM4644-1
VOUT −1
Top Feedback Resistor
Integrated External (to be added on 0.6
from VOUT-to-VFB
60.4k 0.5% PCB)
(one resistor per channel)
Resistor
Table 1. VFB Resistor Table vs Various Output Voltages Application
General To Interface with
V
Applications PMBus power system
OUT (V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
management supervisory RFB(BOT) (k) Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25 ICs such as the LTC2975 Rev. G 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Block Diagram Decoupling Requirements Operation Applications Information Typical Applications Package Description Revision History Package Photo Design Resources Related Parts