MIC2182 datasheet(17/40 Pages) MICROCHIP

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DS20006644A-page 17

MIC2182

The maximum current limit is:

EQUATION 4-6:

The current-sense pins CSH (Pin 8) and VOUT (Pin 9)

are noise sensitive due to the low signal level and high

input impedance. The PCB traces should be short and

routed close to each other. A small (1 nF to 0.1 µF)

capacitor across the pins will attenuate high frequency

switching noise.

When the peak inductor current exceeds the

current-limit threshold, the current-limit comparator, in

the Block Diagrams, turns off the high-side MOSFET

for the remainder of the cycle. The output voltage drops

as additional load current is pulled from the converter.

When the output voltage reaches approximately 0.95V,

the circuit enters frequency-foldback mode and the

oscillator frequency will drop to 60 kHz while

maintaining the peak inductor current equal to the

nominal 100 mV across the external current-sense

resistor. This limits the maximum output power

delivered to the load under a short circuit condition.

4.3

Reference, Enable, and UVLO

Circuits

The output drivers are enabled when the following

conditions are satisfied:

• The VDD voltage (Pin 11) is greater than its

undervoltage threshold (typically 4.2V)

• The voltage on the enable pin is greater than the

enable UVLO threshold (typically 2.5V)

The internal bias circuit generates a 1.245V bandgap

reference voltage for the voltage error amplifier and a

5V VDD voltage for the gate drive circuit. The reference

voltage in the fixed-output-voltage versions of the

MIC2182 is buffered and brought to Pin 7. The VREF pin

should be bypassed to GND (Pin 4) with a 0.1 µF

capacitor. The adjustable version of the MIC2182 uses

Pin 7 for output voltage sensing. A decoupling

capacitor on Pin 7 is not used in the adjustable output

voltage version.

FIGURE 4-6:

Minimum PWM-Mode Load Inductor Current for PWM Operation.

FIGURE 4-7:

Maximum Skip-Mode-Load Inductor Current.

The enable pin (Pin 6) has two threshold levels,

allowing the MIC2182 to shut down in a low current

mode, or turn off output switching in UVLO mode. An

enable pin voltage lower than the shutdown threshold

turns off all the internal circuitry and reduces the input

current to typically 0.1 µA.

If the enable pin voltage is between the shutdown and

UVLO thresholds, the internal bias, VDD, and reference

voltages are turned on. The soft-start pin is forced low

by an internal discharge MOSFET. The output drivers

are inhibited from switching and remain in a low state.

Raising the enable voltage above the UVLO threshold

of 2.5V allows the soft-start capacitor to charge and

enables the output drivers.

Either of two UVLO conditions will pull the soft-start

capacitor low.

• When the VDD drops below 4.1V

• When the enable pin drops below the 2.5V

threshold

4.4

MOSFET Gate Drive

The MIC2182 high-side drive circuit is designed to

switch an N-channel MOSFET. Referring to the Block

Diagrams, a bootstrap circuit, consisting of D2 and

CBST, supplies energy to the high-side drive circuit.

Capacitor CBST is charged while the low-side MOSFET

is on and the voltage on the VSW pin (Pin 15) is

approximately 0V. When the high-side MOSFET driver

is turned on, energy from CBST is used to turn the

high-side MOSFET on. As the MOSFET turns on, the

voltage on the VSW pin increases to approximately VIN.

Diode D2 is reversed biased and voltage at the BST pin

ILIM MAX





135mV

RCS

-----------------

IOUT(MAXSKIP)

Inductor

Current

35mV THRESHOLD

ACROSS RCS.

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