LTM4664 datasheet(28/138 Pages) AD

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FAULT PROTECTION AND THERMAL SHUTDOWN

The LTM4664A divider stages monitor system voltage,

current and temperature for faults. The stage 1 or 2 stops

switching and pulls down its FAULT pin when fault condi-

tions occur. To clear voltage faults, the VOUTn pin voltage

has to be within the programmed window around half of

the VINSn voltage or the VINSn and VOUTSn voltages must

be lower than 1V and 0.5V respectively. To clear current

faults, the voltage drop from INSNSn+ pin to INSNSn– pin

has to be lower than 50mV. To clear temperature faults,

the IC temperature has to be lower than 165°C. The FAULT

pin is allowed to be pulled up by external resistors to

voltages up to 60V. It can also be used to control discon-

nect FETs that isolates the input and output during fault

conditions. See Figure 1 block diagram.

HIGH SIDE CURRENT SENSING

For over current protection, the LTM4664A uses a sensing

resistor RSENSE to monitor the current. The sensing resis-

tor has to be placed at the drain of the very top MOSFET

M1. See Typical Application section for examples. In most

applications, the current through the sensing resistor is a

pulse current and the peak value is much higher than the

average load current. An internal RC filter on the ISENSE–

pin, with a time constant lower than switching frequency,

is used to set the precision average current protection. If

over current protection is not desired, short the ISENSE+

and ISENSE– pins together and connect them to the drain

of top MOSFET M1 directly. This is done in stage 2 since

stage 1 already monitors for current faults. See Figure 1.

FREQUENCY SELECTION

The selection of switching frequency is a trade-off

between efficiency and component size. Low frequency

operation increases efficiency by reducing MOSFET

switching losses, but requires larger capacitance to main-

tain low output ripple voltage and low output impedance.

The FREQSn pin can be used to program the controller’s

operating frequency from 100kHz to 1MHz. There is a

precision 10μA current flowing out of the FREQSn pin, so

the user can program the controller’s switching frequency

Each of these can supply a peak current of 150mA. No

matter what type of bulk capacitor is used, an additional

0.1μF ceramic capacitor placed directly adjacent to the

INTVCCSn and GND pins is highly recommended. Good

bypassing is needed to supply the high transient currents

required by the MOSFET gate drivers. These high input

voltages along with the power MOSFETs being driven at

high frequencies may cause the maximum junction tem-

perature rating for the LTM4664A to be exceeded. The

INTVCCSn current, which is dominated by the gate charge

current, may be supplied by either the 5.5V linear regu-

lator from VINSn or the linear regulator from EXTVCCSn.

When the voltage on the EXTVCCSn pin is less than 6.5V,

the linear regulator from VINSn is enabled. Power dissipa-

tion for the internal controller in this case is highest and

is equal to VINSn • IINTVCCSn. The gate charge current is

dependent on operating frequency. This is why it is highly

recommended to use the VOUT2 voltage to supply power

to the EXTVCCS1,2 pins.

START-UP AND SHUTDOWN

The LTM4664A divider stages are in shutdown mode

when their RUNS pins are pulled down and lower than

1.1V. In this mode, most internal circuitry is turned off

including the INTVCCS1,2 regulators and the 4:1 divider

consumes less than 200μA current per stage. All gates

drives are actively pulled low to turn off the external

power MOSFETs in shutdown. Releasing RUNS1,2 allows

an internal 1μA current to pull up these pins and enable

the controller stage. Once the RUNS1,2 pin raises above

1.22V, an additional 5μA is flowing out of the respective

pin. Alternately, the RUNS pin may be externally pulled

up or driven directly by logic. Do not exceed the Absolute

Maximum Rating of 6V on these pins. After RUNS1,2 pin

is released and the INTVCCS1,2 voltage passes UVLO,

then that particular stage starts up and monitors the VINn

and VOUTn voltage continuously. The LTM4664A divider

stages start switching only if the VOUTn voltage is close

to half of the VINSn voltage or both VOUTn and VINSn volt-

ages are close to GND. In voltage divider applications,

VOUT1,2 is pre-balanced to half the VINS1,2 voltage and the

LTM4664A divider stages may start up with capacitors at

different initial conditions and balancing will be invoked

if necessary.

LTM4664A

Rev. 0

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4:1 DIVIDER OPERATION

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