34652 datasheet(12/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34652

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

START-UP SEQUENCE

When power is first applied to the 34652 by connecting the

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

positive voltage rail, the 34652 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 Power-ON Reset (POR) and once the Undervoltage

Lockout (UVLO) threshold is cleared, the 34652 checks for

external components on four pins—the UV, the ILIM, the

ICHG, and 128

µs later the OV—to set the levels of the

Undervoltage Threshold, the Overcurrent Limit, the Charging

Current Limit, and the Overvoltage Threshold, respectively.

The device also checks for external components on the

TIMER pin to decide on the Start-Up and Retry Delay Timer

value, and the device keeps checking the TIMER pin

continuously throughout the operation.

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 either a default or a

user-programmable 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 either a default or user-programmable

value.

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

This threshold is either a default or user-programmable

value.

• Die temperature is less than 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 34652 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

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