MIC2186

Micrel

MIC2186

10

July 2002

Conditions:

MOSFET gate

drive @ 10V/div

Switch Node Voltage

(MOSFET Drain)

@10V/div

VOUT Ripple Voltage

@50mV/div

Inductor Current @

1A/div

PWM Mode Waveforms

TIME (1

µs/div)

Figure 5 - PWM mode waveforms

The MIC2186 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

I_inductor

I_inductor

Gate Driver

I_inductor

Voltage

Divider

Gate Drive at OutN

Figure 6: PWM Control Loop

A block diagram of the MIC2186 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

required in current mode control to prevent unstable opera-

tion 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 COMP pin (pin 4) to provide access to the output of the

error amplifier. This allows 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

MOSFET and ground (Rsense in Figure 1). Exceeding the

current limit threshold will immediately terminate the gate

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

operate at a reduced duty cycle, which lowers 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 to ground. Only

the impedance of the source and components limits the

current.

The mode of operation (continuous or discontinuous), the

minimum input voltage, maximum output power and the

minimum value of the current limit threshold determine the

value of the current sense resistor. Discontinuous mode is

where all the energy in the inductor is delivered to the output

at each switching cycle. Continuous mode of operation

occurs when current always flows in the inductor, during both

the low-side MOSFET on and off times. The equations below

will help to determine the current sense resistor value for

each operating mode.

The critical value of output current in a boost converter is

calculated below. The operating mode is discontinuous if the

output current is below this value and is continuous if above

this value.

I

VV

V

2fs L

V

CRIT

IN

2

O

IN

O

2

=

×−

×× ×

η

where:

η is the efficiency of the boost converter

Vin is the minimum input voltage

L is the value of the boost inductor

Fs is the switching frequency

Vo is the output voltage

Maximum Peak Current in Discontinuous Mode:

The peak inductor current is:

I

2I

V

V

Lfs

IND(pk)

OO

IN

=

××

− ×

()

×

η

where:

Io is the maximum output current

Vo is the output voltage

Vin is the minimum input voltage

L is the value of the boost inductor

fs is the switching frequency

η is the efficiency of the boost converter

The maximum value of current sense resistor is: