LTM4680 datasheet(18/36 Pages) AD

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LTM4650-2

Rev. 0

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APPLICATIONS INFORMATION

Power Good

The PGOOD pins are open drain pins that can be used to

monitor valid output voltage regulation. This pin monitors

a 10% window around the regulation point. A resistor can

be pulled up to a particular supply voltage no greater than

6V maximum for monitoring.

Stability Compensation

An external RC filtering circuit is required to add from

COMP to SGND to achieve fast Type II control loop

compensation. Table 4 is provided for most application

requirements. The Analog Devices µModule power design

tool (LTpowerCAD) will be provided for other control

loop optimization.

Run Enable

The RUN pins have an enable threshold of 1.4V maximum,

typically 1.25V, with 150mV of hysteresis. They control the

turn on each of the channels and INTVCC. These pins can be

pulled up to VIN for 5V operation, or a 5V Zener diode can be

placed on the pins, and a 10k to 100k resistor can be placed

up to higher than 5V input to enable the channels. The

RUN pins can also be used for output voltage sequencing.

In parallel operation, the RUN pins can be tied together

and controlled from a single control. See the Typical

Applications circuits in Figure 24.

INTVCC and EXTVCC

The LTM4650-2 module has an internal 5V low dropout

regulator that is derived from the input voltage. This regu-

lator is used to power the control circuitry and the power

MOSFET drivers. This regulator can source up to 70mA,

and typically uses ~30mA for powering the device at the

maximum frequency. This internal 5V supply is enabled

by either RUN1 or RUN2.

EXTVCC allows an external 5V supply to power the

LTM4650-2 and reduces power dissipation from the inter-

nal low dropout 5V regulator. The power loss savings can

be calculated by:

(VIN – 5V) • 30mA = PLOSS

EXTVCC has a threshold of 4.7V for activation, and a maxi-

mum rating of 6V. When using a 5V input, connect this

5V input to EXTVCC also to maintain a 5V gate drive level.

EXTVCC must sequence on after VIN, and EXTVCC must

sequence off before VIN.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is provided

to sense low output voltages accurately at the remote

load points. This is especially true for high current loads.

The amplifier can be used on one of the two channels, or

on a single parallel output. It is very important that the

DIFFP and DIFFN are connected properly at the output,

and DIFFOUT is connected to either VOUTS1 or VOUTS2.

In parallel operation, the DIFFP and DIFFN are connected

properly at the output, and DIFFOUT is connected to

one of the VOUTS pins. See the parallel schematics in

Figure 25 and see Figure 4.

SW Pins

The SW pins are generally for testing purposes by moni-

toring these pins. These pins can also be used to dampen

out switch node ringing caused by LC parasitic in the

switched current paths. Usually a series R-C combination

is used called a snubber circuit. The resistor will dampen

the resonance, and the capacitor is chosen only to affect

the high-frequency ringing across the resistor. If the stray

inductance or capacitance can be measured or approxi-

mated, then a somewhat analytical technique can be used

to select the snubber values. The inductance is usually

easier to predict. It combines the power path board induc-

tance in combination with the MOSFET interconnect bond

wire inductance.

First, the SW pin can be monitored with a wide bandwidth

scope with a high-frequency scope probe. The ring fre-

quency can be measured by its value. The impedance Z

can be calculated:

ZL = 2πfL,

where f is the resonant frequency of the ring, and L is the

total parasitic inductance in the switch path. If a resistor

is selected that is equal to Z, then the ringing should be

dampened. The snubber capacitor value is chosen so that

its impedance is equal to the resistor at the ring frequency.

Calculated by: ZC = 1/(2πfC). These values are a good

place to start with. Modifications to these components

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