MIC2182 datasheet(13/28 Pages) MICREL

13 / 28 page

June 2000

MIC2182

Micrel

Switching from PWM to Skip Mode

The current sense amplifier in Figure 3 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

12mV. This is shown in Figure 7b. The average output current

at this transition level for is calculated below.

0.012

OUT(skipmode)

where:

0.012 = threshold voltage of the internal comparator

CS = current-sense resistor value

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 100% (continuous pulses). Increasing the

current past this point will cause the output voltage will 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 35mV/R

(see Figure 7b). The maximum average output current in skip

mode is the average value of the inductor waveform:

0.5

35mV

OUT(max skipmode)

=×

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 10uA current source

on pin 2. The threshold on pin 2 is 2.5V. The delay for a typical

1nF capacitor is:

1nF

2.5V

10 A

250 s

delay

PWM

threshold

source

where:

PWM = capacitor connected to pin 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 (R

CS in Figure 2.). The cur-

rent-limit threshold is 100mV+35mV –25mV. The current-

sense resistor must be sized using the minimum current-limit

threshold. The external components must be designed to

withstand the maximum current limit. The current-sense

resistor value is calculated by the equation below:

75mV

OUT(max)

The maximum output current is:

135mV

OUT(max)

The current-sense pins CSH (pin 8) and V

OUT (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 (1nF 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 Figure 2, 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 approxi-

mately 0.95V, the circuit enters frequency-foldback mode

and the oscillator frequency will drop to 60kHz while maintain-

ing the peak inductor current equal to the nominal 100mV

across the external current-sense resistor. This limits the

maximum output power delivered to the load under a short

circuit condition.

Reference, Enable and, UVLO Circuits

The output drivers are enabled when the following conditions

are satisfied:

• The V

DD 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 refer-

ence voltage for the voltage error amplifier and a 5V V

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 V

REF 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.

ILIM(skip)

Inductor

Current

35mV THRESHOLD

ACROSS RCS.

Figure 7a. Maximum Skip-Mode-Load Inductor Current

IMIN(PWM)

Inductor

Current

12mV THRESHOLD

OF AVERAGE VOLTAGE

ACROSS RCS.

Figure 7b. Minimum PWM-Mode-Load Inductor Current for PWM Operation

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