MIC33M350 datasheet(19/32 Pages) MICROCHIP

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DS20006348B-page 19

MIC33M350

4.10

Switching Frequency

The switching frequency of the MIC33M350 is

determined by the internal On-Time (TON) calculation.

For an input voltage of 5V and an output voltage of 1V,

the typical value of TON is 180 ns. The resulting

switching frequency can be estimated by Equation 4-3.

EQUATION 4-3:

Equation 4-3 is only valid in continuous conduction

mode and for a lossless converter. In practice, losses

cause an increase of the switching frequency

compared to the ideal case. As the load current

increases, losses increase too and so does the

switching frequency.

The on-time calculation is adaptive, in that the TON

value is modulated based on the input voltage and on

the target output voltage to stabilize the switching

frequency against their variations. Losses are not

accounted for.

4.11

Undervoltage Protection (UVLO)

Undervoltage protection ensures that the IC has

enough voltage to bias the internal circuitry properly

and provide sufficient gate drive for the power

MOSFETs. When the input voltage starts to rise, both

power MOSFETs are off and the power good output is

pulled low. The IC starts at approximately 2.225V and

has a nominal 153 mV of hysteresis to prevent

chattering between the UVLO high and low states.

4.12

Overtemperature Fault

The MIC33M350 monitors the die junction temperature

to keep the IC operating properly. If the IC junction

temperature exceeds +165°C, both power MOSFETs

are immediately turned off. The IC is allowed to restart

when the die temperature falls below +143°C.

During recovery from a thermal shutdown event, if the

regulator hits another thermal shutdown event or a

current limit event causing hiccup before Power Good

can be achieved, the controller resets again. If this

happens more than four times in a row, then the part

enters the Latch-Off state, which turns off both

MOSFETs permanently. The MIC33M350 part does not

restart again unless the input power is cycled. This

Latch-Off feature eliminates the thermal stress on the

MIC33M350 during a persistent Fault event.

4.13

Safe Start-up Into a Pre-Biased

Output

The MIC33M350 is designed for safe start-up into a

pre-biased output in forced PWM. This feature

prevents high negative inductor current flow in a

pre-bias condition, which can damage the IC. This is

achieved by not allowing forced PWM until the control

loop commands eight switching cycles. After eight

cycles, the low-side negative current limit is switched

from 0A to -3A. The cycle counter is reset to zero if the

enable pin is pulled low, or an input undervoltage

condition or any other Fault is detected.

4.14

Current Limiting

The MIC33M350 regulator uses both high-side and

low-side current sense for current limiting. When the

high-side current sense threshold is reached, the

high-side MOSFET is turned off and the low-side

MOSFET is turned on. The low-side MOSFET stays on

until the current falls to 80% of the high-side current

threshold value, then the high side can be turned on

again. If the overload condition lasts for more than four

cycles, the MIC33M350 enters hiccup current limiting

and both MOSFETs are turned off. There is a 1 ms

cool-off period before the MOSFETs are allowed to be

turned on. If the regulator has another hiccup event

before it reaches the Power Good threshold on restart,

turn both MOSFETs off again and wait for 1 ms. If this

happens more than three times in a row, then the part

enters the Latch-Off state, which turns off both

MOSFETs permanently, unless the part is reset by

cycling the input power.

4.15 Thermal Considerations

Although the MIC33M350 is capable of delivering up to

3A under load, the package thermal resistance and the

device internal power dissipation may dictate some

limitations to the continuous output current.

If operated above the rated junction temperature,

electrical parameters may drift beyond characterized

specifications. The MIC33M350 is protected under all

circumstances by thermal shutdown.

VIN (V)

VOUT (V)

TON

50.6

110

1180

1.8

340

2.5

490

3.3

610

3.3

270

fSW

VOUT VIN TON

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