34652 datasheet(16/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34652

FUNCTIONAL DEVICE OPERATION

PROTECTION FEATURES

Figure 15 shows how the 34652 uses the start-up timer to

make sure that the input voltage is above the undervoltage

falling threshold. The 34652 was in normal operation before

Point A. At Point A an undervoltage fault occurs. Then the

fault is cleared at Point B, and the 34652 initiates a start-up

sequence. Before the end of the start-up timer another

undervoltage fault occurs at Point C, so the 34652 does not

turn on the Power MOSFET. At Point D the fault is cleared

again for the length of the start-up timer. The 34652 turns on

the Power MOSFET and starts the charging process (refer to

Charging Process, pages 12–13).

Figure 15. Start-Up Timer Protection Against

Undervoltage Faults

OVERVOLTAGE

When the voltage on the OV pin exceeds the overvoltage

rising threshold for more than 1.0 ms, an overvoltage fault is

detected and one of the start-up conditions (list on page 12)

is violated. The 34652 turns off the Power MOSFET and

deactivates the power good output signals, thus disabling the

load. The 34652 stays in this state until the voltage on the OV

pin falls below the overvoltage falling threshold for more than

1.0 ms, signaling that the supply voltage is in the normal

operation range. Then a new start-up sequence initiates as

described on page 12.

The overvoltage detection circuit is also equipped with a

2.0 V hysteresis when in default mode. The hysteresis value

depends on the overvoltage detection threshold and can be

calculated as follows:

VOVHY = VOV(RISING) * [1 - (1.3 V - VOVCHY) / 1.3 V]

The waveforms for an overvoltage fault are shown in

Figure 16, page 16.

Figure 16. Overvoltage Fault

THERMAL SHUTDOWN

The thermal shutdown feature helps protect the internal

Power MOSFET and circuitry from excessive temperatures.

During start-up and thereafter during normal operation, the

34652 monitors the temperature of the internal circuitry for

excessive heat. If the temperature of the device exceeds the

thermal shutdown temperature of 160

°C, one of the start-up

conditions (list on page 12) is violated, and the device turns

off the Power MOSFET and deactivates the power good

output signals. Until the temperature of the device goes

below 135

°C, a new start-up sequence will not be initiated.

This feature is an advantage over solutions with an external

Power MOSFET, because it is not easy for a device with an

external MOSFET to sense the temperature quickly and

accurately. The thermal shutdown circuit is equipped with a

µs filter.

Thermal design is critical to proper operation of the 34652.

The typical RDS(ON) of the internal Power MOSFET is

0.144

Ω at room ambient temperature and can reach up to

0.251

Ω at high temperatures. The thermal performance of

the 34652 can vary depending on many factors, among them:

• The ambient operating temperature (TA).

• The type of PC board—whether it is single layer or multi-

layer, has heat sinks or not, etc.—all of which affects the

value of the junction-to-ambient thermal resistance (RθJA).

• The value of the desired load current (ILOAD).

When choosing an overcurrent limit, certain guidelines

need to be followed to make sure that if the load current is

running close to the overcurrent limit the 34652 does not go

into thermal shutdown. It is good practice to set the

parameters so that the resulting maximum junction

temperature is below the thermal shutdown temperature by a

safe margin.

Equation 1, on the following page can be used to calculate

the maximum allowable overcurrent limit based on the

maximum desired junction temperature or vice versa.

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