LTC2977 datasheet(66/138 Pages) AD

66 / 138 page

LTM4664A

Rev. 0

For more information www.analog.com

MINIMUM ON-TIME CONSIDERATIONS

Minimum on-time, tON(MIN), is the smallest time duration

that the LTM4664A PSM is capable of turning on the top

MOSFET. It is determined by internal timing delays and the

gate charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN) <

VOUTn

VINn • fOSC

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTM4664A is 60ns.

VARIABLE DELAY TIME, SOFT-START AND OUTPUT

VOLTAGE RAMPING

The LTM4664A PSM must enter its run state prior to soft-

start. The RUN_Cn pins are released after the part initial-

izes, and VINS3 is greater than the VIN_ON threshold and

Stage 2 PGood pin releases the RUN_Cn pins. If multiple

LTM4664As are used in an application, they should be

configured to share the same RUN_Cn pins. They all hold

their respective RUN_Cn pins low until all devices initialize

and VINS3 exceeds the VIN_ON threshold for all devices.

The SHARE_CLK pin assures all the devices connected to

the signal use the same time base.

After the RUN_Cn pin releases, the controller waits for the

user-specified turn-on delay (TON_DELAYn) prior to ini-

tiating an output voltage ramp. Multiple LTM4664As and

other LTC parts can be configured to start with variable

delay times. To work correctly, all devices use the same

timing clock (SHARE_CLK) and all devices must share

the RUN_Cn pin.

This allows the relative delay of all parts to be synchro-

nized. The actual variation in the delay will be dependent

on the highest clock rate of the devices connected to the

SHARE_CLK pin (all Analog Devices ICs are configured

to allow the fastest SHARE_CLK signal to control the

timing of all devices). The SHARE_CLK signal can be

±10% in frequency, thus the actual time delays will have

some variance.

Soft-start is performed by actively regulating the load

voltage while digitally ramping the target voltage from 0V

to the commanded voltage set point. The rise time of the

voltage ramp can be programmed using the TON_RISEn

command to minimize inrush currents associated with the

start-up voltage ramp. The soft-start feature is disabled by

setting TON_RISEn to any value less than 0.250ms. The

LTM4664A PSM performs the necessary math internally to

assure the voltage ramp is controlled to the desired slope.

However, the voltage slope can not be any faster than the

VOUTn fundamental limits of the power stage. The number

of tON(MIN) < steps in the ramp is equal to TON_RISE/0.1ms.

Therefore, the shorter the TON_RISEn time setting, the

more discrete steps in the soft-start ramp appear.

The LTM4664A PSM PWM always operates in discon-

tinuous mode during the TON_RISEn operation. In dis-

continuous mode, the bottom MOSFET (MBn) is turned

off as soon as reverse current is detected in the inductor.

This allows the regulator to start up into a prebiased load.

There is no analog tracking feature in the LTM4664A PSM;

however, two outputs can be given the same TON_RISEn

and TON_DELAYn times to achieve ratiometric rail track-

ing. Because the RUNn pins are released at the same time

and both units use the same time base (SHARE_CLK), the

outputs track very closely. If the circuit is in a PolyPhase

configuration, all timing parameters must be the same.

DIGITAL SERVO MODE

For maximum accuracy in the regulated output voltage,

enable the digital servo loop by asserting bit 6 of the

MFR_PWM_MODE command. In digital servo mode, the

LTM4664A PSM will adjust the regulated output voltage

based on the ADC voltage reading. Every 90ms the dig-

ital servo loop will step the LSB of the DAC (nominally

1.375mV or 0.6875mV depending on the voltage range

bit) until the output is at the correct ADC reading. At pow-

er-up this mode engages after TON_MAX_FAULT_LIMIT

unless the limit is set to 0 (infinite). If the TON_MAX_

FAULT_LIMIT is set to 0 (infinite), the servo begins

after TON_RISE is complete and VOUT has exceeded the

DUAL 25A/30A PSM APPLICATIONS INFORMATION

LTC2977 Datasheet(PDF) 66 Page - Analog Devices

LTC2977 Datasheet(HTML) 66 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL