MIC2185

Micrel, Inc.

MIC2185

10

October 2005

PWM Operation

PWM ModeWaveform

TIME (1µs/div)

Switch NodeVoltage

(Low Side FET Drain)

5V/div

V

High Side FET

Gate Drive

5V/div

Low Side FET

Gate Drive

5V/div

Inductor Current

1A offset; 0.5A/div

V

200mV/div

V

I

Figure 5 - PWM mode waveforms

Figure5showstypicalwaveformsforPWMmodeofoperation.

The gate drive signal turns on the external low side MOSFET,

Q1, allowing the inductor current to ramp up. When the low

side MOSFET turns off and the high side MOSFET, Q2, turns

on, current flowing in the inductor forces the MOSFET drain

voltage to rise until the is clamped at approximately the output

voltage. The MIC2185 uses current mode control to improve

output regulation and simplify compensation of the control

loop. Current mode control senses both the output voltage

(outer loop) and the inductor current (inner loop). It uses the

inductor current and output voltage to determine the duty

cycle (D) of the buck converter. Sampling the inductor current

effectively removes the inductor from the control loop, which

simplifies compensation. A simplified current mode control

diagram is shown in figure 6.

I_inductor

T

ON

T

PER

V

COMP

Gate Drive at OUTN

I_inductor

I_inductor

Gate Driver

I_inductor

Voltage

Divider

Figure 6. PWM Control Loop

A block diagram of the MIC2185 PWM current mode control

loop is shown in Figure 1. The inductor current is sensed by

measuring the voltage across a resistor, Rsense. The current

sense amplifier buffers and amplifies this signal. A ramp is

added to this signal to provide slope compensation, which is

requiredincurrentmodecontroltopreventunstableoperation

at duty cycles greater than 50%.

A transconductance amplifier is used as an error amplifier,

which compares an attenuated output voltage with a refer-

ence voltage. The output of the error amplifier is compared

to the current sense waveform in the PWM block. When the

current signal rises above the error voltage, the comparator

turns off the low side drive. The error signal is brought out to

the COMPpin (pin 4) allowing the use of external components

to stabilize the voltage loop.

Current Sensing and Overcurrent Protection

The inductor current is sensed during the switch on time by

a current sense resistor located between the source of the

the current limit threshold will immediately terminate the

gate drive of the N-channel MOSFET. This forces the Q1 to

operate at a reduced duty cycle, which reduces the output

voltage.

In a boost converter, the overcurrent limit will

not protect the power supply or load during a severe

overcurrent condition or short circuit condition. If the

output is short-circuited to ground, current will flow from the

input, through the inductor and output diode,D1, to ground.

Only the impedance of the source and components limits

the current.

The minimum input voltage, maximum output power and

the minimum value of the current limit threshold determine

the value of the current sense resistor. The two switch, syn-

chronous operation of the MIC2185 forces the converter to

always operate in the continuous mode because current can

flow both ways through the high side P-channel MOSFET.

The equations below will help to determine the current sense

resistor value.

Maximum Peak Current

The peak inductor current is equal to the average inductor

current plus one half of the peak to peak inductor current.

The peak inductor current is:

I

I

1

2

I

I

V

I

V

V

V

V

2 V

fs L

IND(pk)

IND(ave)

IND(pp)

IND(pk)

O

O

IN

L

O

IN

O

=

+ ×

=

×

×

+

×

−

×

(

)

(

)

×

× ×

η

η

where:

L is the value of the boost inductor

η

is the efficiency of the boost converter

However, the voltage drop across the inductor winding

resistance and low side MOSFET on-resistance must be

accounted for at the lower input voltages that the MIC2185

can operate at: