LTM4664 datasheet(27/138 Pages) AD

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LTM4664A

Rev. 0

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4:1 DIVIDER OPERATION

Figure 3. Stage 1 MOSFET Switching Waveforms

VGS

~ 50% DUTY CYCLE

PHASE 1

PHASE 2

PHASE 1

PHASE 2

4664A F03

4:1 DIVIDER DESCRIPTION

The LTM4664A incorporates a high performance 4:1

switched capacitor divider that divides down the input

voltage by a factor of four over an input voltage range of 30V

to 58V. This 4:1 divider then powers a dual 25A/30A PMBus

compliant core power rails that can regulate from 0.5V to

1.5V at up to 25A, per channel—and from 0.5V to 1.2V at

up to 30A per channel. The LTM4664A will convert this

high input range down directly to the low output voltages.

MAIN CONTROL

The LTM4664A internal 4:1 divider utilizes two constant

frequency, open loop switched capacitor/charge pump

stages for high voltage step down. The conversion effi-

ciency is very high at ~ 99% for stage 1, and ~98.6% effi-

cient for stage 2. Refer to Figure 1 for the block diagram.

In stage 1 steady state operation, the N-channel MOSFETs

M1 and M3 are turned on and off in the same phase with

around 50% duty cycle at a pre-programmed 100kHz

switching frequency. The N-channel MOSFETs M2 and

M4 are turned on and off complementary to MOSFETs M1

and M3. The gate drive waveforms are shown in Figure 3.

During phase 1, M1 and M3 are on and the flying capaci-

tor CFLY1 is in series with COUT1. During phase 2, M2 and

M4 are on and CFLY1 is in parallel with COUT1. The VOUT1

voltage is always close to half of the top voltage at the

drain of MOSFET M1 (refer to GND pin) and in steady state

and it is not sensitive to variable loads due to the very

low impedance at its output. Stage 2 operates exactly the

same way, the N-channel MOSFETs M5 and M7 are turned

on and off in the same phase with around 50% duty cycle

at a pre-programmed 300kHz switching frequency. The

N-channel MOSFETs M6 and M8 are turned on and off

complementary to MOSFETs M5 and M7. The main input

voltage rail is connected to VINS1, and VOUT1 is connected

directly into the VINS2. The LTM4664A front end divider

stages do not regulate the output voltage with a closed-

loop feedback system. However, it stops switching when

fault conditions occur, such as VOUT pin overvoltage or

undervoltage, an overcurrent event, or an over tempera-

ture protection event.

The VOUT2 (VINS1/4) is connected to the VINS3_Cn which is

the inputs to the dual 25A/30A PMBus channels. This will

be discussed in more detail in the dual 25A/30A operation

section. There is a secondary fault protection comparator

circuit that can be used to monitor VOUT2 for over voltage.

This will be discussed in more detail in the application

section. The LTM4664A can operate over a 30V to 58V

input range that does have abrupt input voltage changes

that move quicker than a few milliseconds after reaching

steady state. See the window comparator section.

INTVCCS1,2/EXTVCCS1,2 POWER

Power for the quad N-channel MOSFET drivers and most

other internal circuitry is derived from the INTVCCSn pin.

Normallyaninternal5.5VlinearregulatorsuppliesINTVCCS1,2

power from either VINS1 or VINS2 as indicated in Figure 1.

Both of these input supplies have high input voltage, and

increases power loss due to the LDO drop. An optional

external voltage source on EXTVCCS1,2 pin enables a sec-

ond 5.5V linear regulator and supplies INTVCCS1,2 power

from the EXTVCCS1,2 pin. To enable this more efficient

second regulator, VINS1,2 has to be higher than 7V and

the EXTVCCS1,2 pin voltage has to be higher than 6.5V.

Do not exceed 40V on the EXTVCCSn pin. Figure 1 shows

the VOUT2 supply (12V) connected to both EXTVCCS1 and

EXTVCCS2 to lower power loss in the LDO after startup.

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