LTM4618EVPBF datasheet(12/24 Pages) LINER

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LTM4618

4618f

The LTM4618 can be synchronized from 400kHz to 780kHz

with an input clock that has a high level above 2.0V and a

low level below 0.8V. The 400kHz low end operation limit

is put in place to limit inductor ripple current. See the

Typical Applications section for synchronization examples.

The LTM4618 minimum on-time is limited to about 90ns.

Guardband the on-time to 110ns. The on-time can be

calculated as:

tON(MIN) =

FREQ

•

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Soft-Start and Tracking

LTM4618 has the ability to either soft-start by itself with a

capacitor or track the output of an external supply. When

the module is conﬁgured to soft-start by itself, a capacitor

should be connected to its TK/SS pin. When the module

is in the shutdown state, the TK/SS pin is actively pulled

to ground.

Once the RUN pin voltage is above 1.22V, the module pow-

ers up. Then a soft-start current of 1.3μA starts to charge

its soft-start capacitor. Note that soft-start or tracking is

achieved not by limiting the maximum output current of

the controller but by controlling the output ramp voltage

according to the ramp rate on the TK/SS pin. Current

foldback is disabled during this phase to ensure smooth

soft-start or tracking. The soft-start or tracking range is

deﬁned as the voltage range from 0V to 0.8V on the TK/SS

pin. The total soft-start time can be calculated as:

tSOFT-START =

0.8V •CSS

1.3µA

Output voltage tracking can be programmed externally

using the TK/SS pin. The master voltage is divided down

with an external resistor divider that is the same as the

slave’s feedback divider to implement coincident tracking.

The LTM4618 uses an accurate 60.4k resistor internally

for the top feedback resistor. Figure 4 shows an example

of coincident tracking.

VOUT(SLAVE) = 1+

⎛

⎝⎜

⎞

⎠⎟ •VTRACK

APPLICATIONS INFORMATION

VTRACK is the track ramp applied to the slave’s TK/SS

pin. VTRACK has a control range of 0V to 0.8V. When the

master’s output is divided down with the same resistor

values used to set the slave’s output, then the slave will

coincident track with the master until it reaches its ﬁnal

value. The master will continue to its ﬁnal value from the

slave’s regulation point.

Ratiometric modes of tracking can be achieved by select-

ing different divider resistor values to change the output

tracking ratio. The master output must be greater than the

slave output for the tracking to work. Master and slave

data inputs can be used to implement the correct resistor

values for coincident or ratio tracking.

Figure 4. Output Voltage Coincident Tracking

4618 F04

VIN

COMP

TK/SS

RUN

FREQ

VOUT

VFB

PGOOD

INTVCC EXTVCC

MODE/PLLIN

LTM4618

SGND

PGND

28.7k

COUT

VOUT(SLAVE)

2.5V/6A

60.4k

CIN

VIN

MASTER

OUTPUT

22pF

Figure 5. Coincident Tracking Characteristics

TIME

OUTPUT

VOLTAGE

4618 F05

MASTER OUTPUT

SLAVE OUTPUT

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