34653 datasheet(10/24 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34653

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

START-UP SEQUENCE

When power is first applied to the 34653 by connecting the

VIN pin to the negative voltage rail and the VPWR pin to the

positive voltage rail, the 34653 keeps the Power MOSFET

turned off, deactivates the power good output signals, and

resets the retry counter. If the device is disabled, no further

activities will occur and power-up would not start. If the device

is enabled, it starts to establish an internally regulated supply

voltage required for the internal circuitry. The Power

MOSFET will stay off until the start of the charging process.

After the Power-ON Reset (POR) and once the

Undervoltage Lockout (UVLO) threshold is cleared, the

34653 checks for external components on two pins—ILIM

and ICHG—to set the levels of the Overcurrent Limit and the

Charging Current Limit, respectively. The device then

initiates the start-up timer (Point A in Figure 6) and checks for

the start-up conditions (see next paragraph). The duration of

the timer is a default value. For undervoltage and overvoltage

faults during power up the 34652 retries infinitely until normal

input voltage is attained. If the die temperature ever

increased beyond the thermal shutdown threshold or the

device is disabled, then the start-up timer resets and the retry

counter increments. If after 10 retries the die temperature is

still high and the device is still disabled, the 34652 will not

retry again and the power in the device must be recycled or

the device must be disabled to reset the retry counter.

Figure 6. Start-Up Sequence

Start-Up Conditions

The start-up conditions are as follows:

• Input voltage is below the overvoltage turn-off threshold.

This threshold is a default.

• Input voltage is above the undervoltage turn-off threshold.

This threshold is a default.

• Die temperature is less than the thermal shutdown

temperature.

• Device is enabled.

If the start-up conditions are satisfied for a time equal to

the length of the start-up timer and the retry counter is less

than or equal to 10, the device starts to turn on the Power

MOSFET gradually to control the inrush current that charges

up the load capacitor to eventually switch on the load (Point B

in Figure 6).

Charging Process

When charging a capacitor from a fixed voltage source, a

definite amount of energy will be dissipated in the control

circuit, no matter what the control algorithm is. This energy is

equal to the energy transferred to the capacitor—½CV2.

With this in mind, the Power MOSFET in the 34653 cannot

absorb this pulse of energy instantaneously, so the pulse

must be dissipated over time. To limit the peak power

dissipation in the Power MOSFET and to spread out the

duration of the energy dissipation in the Power MOSFET, the

circuit uses a two-level current approach to controlling the

inrush current and switching on the load as explained in the

following paragraphs.

When the Power MOSFET is turned on, the current limit is

set gradually from 0 A to ICHG (between Points A and B in

Figure 7). The low charging current value and the gradual

rise time of ICHG are either defaults or they can be user

programmable (2.0 ms rise time in the example in Figure 7).

The low charging current value of ICHG is intended to limit the

temperature increase during the load capacitor charging

process, and the gradual rise to ICHG is to prevent transient

dips in the input voltage due to sharp increases in the current.

This prevents the input voltage from drooping due to current

steps acting on the input line inductance, and that in turn

prevents a premature activation of the UV detection circuit.

34653 Datasheet(PDF) 10 Page - Freescale Semiconductor, Inc

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