LTC2977 datasheet(30/138 Pages) AD

30 / 138 page

When switches M2 and M4 are on or it is limited by the

power MOSFET saturation current in the 1st stage, and

M6 and M8 in the 2nd stage:

VCFLYn – VOUTn

RONMn +RONMn

With very low RDS(ON) of the power MOSFETs, the inrush

charge current could easily achieve several hundreds of

Amperes which can be higher than the MOSFET’s Safe

Operating Area (SOA).

The LTM4664A provides a proprietary pre-balance

method to minimize the inrush charging current in voltage

divider applications. The LTM4664A controller detects

the VOUTn pin voltage before switching and compares

it with the VINSn/2 internally. If the VOUTn pin voltage is

much lower than the VINSn/2, a current source will source

95mA current to the VOUTn pin to pull the VOUTn pin up.

If the VOUTn pin voltage is much higher than the VINSn/2,

another current source will sink 50mA from VOUTn pin to

pull the VOUTn pin down.

If the VOUTn pin voltage is close to VINSn/2 and within

the pre-programmed window, both current sources are

disabled and the divider stages start switching. After 68

switching cycles and the VOUTn pin is still within the win-

dow, the FAULTSn pin is released.

For the 4:1 voltage divider with pre-balance startup, the

LTM4664A assumes no load current or very small load

current (less than 50mA) at the VOUTn (output) otherwise

the VOUTn cannot reach VINSn/2 and LTM4664A never

starts up. This no load condition can be achieved by con-

necting the PGOODn pin to the enable pins of the follow-

ing electrical loads. If load current cannot be controlled off

such as resistive loads, a disconnected FETs is required to

disconnect the load to the VOUTn during startup as shown

in the typical applications. The input power source can

operate over the 30V to 58V range, but the supply varia-

tion needs to be constrained to move much slower than

the switching frequency and not exceed the hysteresis set

by the HYS_PRGMSn pin. Large fast voltage excursions

changes will force the 4:1 divider into pre-balance phase.

LTM4664A

Rev. 0

For more information www.analog.com

4:1 DIVIDER APPLICATION INFORMATION

A Typical Application in the Figure 1 block diagram shows

the 4:1 voltage divider circuit. For the 1st stage voltage

divider, the VINS1 input voltage is at the drain of very top

MOSFET M1 and the output voltage is at the VOUT1 pin

which is connected to the source of MOSFET M2 and the

drain of MOSFET M3. The output voltage is around half of

the input voltage in steady state. For the 2nd stage voltage

divider, the VINS2 input voltage is at the drain of very top

MOSFET M5 and the output voltage is at the VOUT2 pin

which is connected to the source of MOSFET M6 and the

drain of MOSFET M7. This completes the 4:1 divider.

For divider applications, if the load current is applied

before startup or heavy resistive loads are connected to

the VOUTn pin, the divider stages may not start up due to

the limited drive current of the pre-balance circuit.

Therefore the PGOODS1 signal is used to sequence on

stage 2 RUN2 pin, and the PGOODS2 pin is used to stage

on the dual 25A/30A regulator.

VOLTAGE DIVIDER PRE-BALANCE BEFORE SWITCHING

In voltage divider applications, the VOUTn voltage should

be always close to VINSn/2 in the steady state. The volt-

ages on the flying capacitors (CFLYn) and VOUTn capacitors

are all very close to each other and equal to the half of

the input voltage. The charging inrush current is mini-

mized during each switching cycle because the voltage

difference between capacitors is small. However, without a

special charging method such as the LTM4664A control-

ler pre-charging circuitry, during start-up or fault condi-

tions such as VOUTn short to GND, the difference between

capacitors can be large and huge charging currents may

be large enough to cause very large MOSFETs currents.

When switches M1 and M3 are on in the 1st stage, and

M5 and M7 are on in the 2nd stage. Ideally, the inrush

charge current is:

VINSn – VCFLYn – VOUTn

RONMn +RONMn

LTC2977 Datasheet(PDF) 30 Page - Analog Devices

LTC2977 Datasheet(HTML) 30 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