LTM4663

9

Rev. A

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+

–

LINEAR

POWER STAGE

PWM SWITCHING

MODE REGULATOR

CONTROL

TEMPERATURE ERROR

AMPLIFIER

CHOPPER 1

+

–

PID COMPENSATION

AMPLIFIER

CHOPPER 2

TAMPOUT

PAMPN

PAMPOUT

TEC

+

–

R

LTM4663 F02

below 0.07V, the controller goes into an ultralow current

state. The current drawn in shutdown mode is 480µA

circuit block, which is always on even when the device is

disabled or shut down. The device can also be enabled

when an external synchronization clock signal is applied

0.07V. Table 1 shows the combinations of the two input

signals that are required to enable the LTM4663.

Table 1. Enable Pin Combinations

EN/SY PIN

STATUS

>2.1V

>0.07V

Enable

Clock

>0.07V

Enable

<0.8V

No Effect*

Shutdown

No Effect*

<0.07V

Shutdown

*No effect means this signal has no effect in shutting down or in enabling the device.

OPERATING FREQUENCY

The LTM4663 has a default 2MHz switching frequency for

the PWM switching regulator output stage. The oscillator

is active when the enabled voltage at the EN/SY pin is

voltage is greater than the shutdown threshold of 0.07V.

FREQUENCY SYNCHRONIZATION AND CLOCK IN

The switching frequency of the LTM4663 can be syn-

chronized to an external clock from 1.85MHz to 3.25MHz

Figure 2. LTM4663 Control Signal Flow Chart

APPLICATIONS INFORMATION

applied to the EN/SY input pin. The clock high level must

be above 2.1V and the clock low level below 0.8V.

SOFT-START

The LTM4663 has an internal soft-start circuit that gen-

erates a ramp with a typical 150ms profile to minimize

inrush current during power-up. The settling time and the

final voltage across the TEC depends on the TEC voltage

required by the control voltage of voltage loop. The higher

the TEC voltage is, the longer it requires to reach the final

output voltage.

When the LTM4663 is first powered up, the linear side

discharges the output of any prebias voltage. As soon

as the prebias is eliminated, the soft-start cycle begins.

During the soft-start cycle, both the PWM and linear out-

puts track the internal soft-start ramp until they reach

linear outputs are then diverge from each other until the

required differential voltage is developed across the TEC

or the differential voltage reaches the voltage limit. The

voltage developed across the TEC depends on the control

point at that moment. Figure 3 shows an example of the

rent overshoot at the point where the TEC voltage starts

to build up.