MIC2182 datasheet(16/40 Pages) MICROCHIP

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MIC2182

DS20006644A-page 16



2022 Microchip Technology Inc. and its subsidiaries

FIGURE 4-5:

Efficiency.

4.1.4

SWITCHING FROM PWM TO SKIP

MODE

The current sense amplifier in Figure 4-1 monitors the

average voltage across the current-sense resistor. The

controller will switch from PWM to skip mode when the

average voltage across the current-sense resistor

drops below approximately 12 mV if the PWM/Skip

mode selection is set to automatic. This is shown in

Figure 4-6. The average output current at this transition

level for is calculated below.

EQUATION 4-2:

4.1.5

SWITCHING FROM SKIP TO PWM

MODE

The frequency of occurrence of the skip-mode current

pulses increase as the output current increases until

the hysteretic duty cycle reaches full CCM duty cycle

(continuous pulses). Increasing the current past this

point will cause the output voltage to drop. The low limit

comparator senses the output voltage when it drops

below 2% of the set output and automatically switches

the converter to PWM mode.

The inductor current in skip mode is a triangular wave

shape a minimum value of 0 and a maximum value of

35 mV/RCS (see Figure 4-7). The maximum average

output current in skip mode is the average value of the

inductor waveform:

EQUATION 4-3:

The PWM pin (Pin 2) is the PWM/Skip mode selection

pin. When the PWM pin is logic level low, the device is

set in forced PWM operation. A capacitor (typically

1 nF) connected across the PWM pin and ground sets

the device to automatic PWM/Skip selection according

to the output current level.

The capacitor on the PWM pin (Pin 2) is discharged

when the IC transitions from skip to PWM mode. This

forces the IC to remain in PWM mode for a fixed period

of time. The added delay prevents unwanted switching

between PWM and skip mode. The capacitor is

charged with a 10 µA current source on Pin 2. The

threshold on Pin 2 is 2.5V. The delay for a typical 1 nF

capacitor is:

EQUATION 4-4:

4.2

Current Limit

The current-limit circuit operates during PWM mode.

The output current is detected by the voltage drop

across the external current-sense resistor (RCS in the

Block Diagrams). The current-limit threshold voltage is

100 mV +35 mV/–25 mV. The current-sense resistor

must be sized using the minimum current-limit

threshold voltage. The external components must be

designed to withstand the maximum current limit. The

current-sense resistor value is calculated by the

equation below:

EQUATION 4-5:

100

0.01

0.1

100

OUTPUT CURRENT (A)

Skip

PW M

IOUT MINPWM





0.012V

RCS

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

Where:

0.012V = Threshold Voltage of the Internal

Comparator

RCS =

Current-Sense Resistor Value

IOUT MAXSKIP





0.5 35mV

RCS

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



tDELAY

CPWM VTH_MODESEL



IPWM_SRC

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

1nF 2.5V



10A

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

250s

Where:

CPWM = Capacitor connected to Pin 2.

VTH_MODESEL = Mode selection threshold voltage

(2.5V typ.)

IPWM_SRC = PWM pin source current (10 µA typ.)

RCS

75mV

IOUT MAX





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



Where:

IOUT(MAX) = Target maximum output current.

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