MIC2177BWM datasheet(9/15 Pages) MICREL

9 / 15 page

Micrel, Inc.

MIC2177

April 2008

M9999-042108

discharged from the inductor and IL1 decreases until the

next switching cycle begins. By varying the P-channel

on-time (duty cycle), the average inductor current is

adjusted to whatever value is required to regulate the

output voltage.

The MIC2177 uses current-mode control to adjust the

duty cycle and regulate the output voltage. Current-

mode control has two signal loops that determine the

duty cycle. One is an outer loop that senses the output

voltage, and the other is a faster inner loop that senses

the inductor current. Signals from these two loops

control the duty cycle in the following way: VOUT is fed

back to the error amplifier which compares the feedback

voltage (VFB) to an internal reference voltage (VREF).

When VOUT is lower than its nominal value, the error

amplifier output voltage increases. This voltage then

intersects the current-sense waveform later in switching

period which increases the duty cycle and average

inductor current. If VOUT is higher than nominal, the error

amplifier output voltage decreases, reducing the duty

cycle.

The PWM control loop is stabilized in two ways. First,

the inner signal loop is compensated by adding a

corrective ramp to the output of the current sense

amplifier. This allows the regulator to remain stable

when operating at greater than 50% duty cycle. Second,

a series resistor-capacitor load is connected to the error

amplifier output (COMP pin). This places a pole-zero

pair in the regulator control loop.

One more important item is synchronous rectification. As

mentioned earlier, the N-channel output MOSFET is

turned on after the P-channel turns off. When the N-

channel turns on, its on-resistance is low enough to

create a short across the output diode. As a result,

inductor current flows through the N-channel and the

voltage drop across; it is significantly lower than a diode

forward voltage. This reduces power dissipation and

improves efficiency to greater than 95% under certain

operating conditions.

To prevent shoot through current, the output stage

employs break-before-make circuitry that provides

approximately 50ns of delay from the time one MOSFET

turns off and the other turns on. As a result, inductor

current briefly flows through the output diode during this

transition.

Skip-Mode Operation

Refer to “Skip-Mode Functional Diagram” which is a

simplified block diagram of the MIC2177 operating in

skip mode and its associated waveforms.

Skip-mode operation turns on the output P-channel at a

frequency and duty cycle that is a function of VIN, VOUT,

and the output inductor value. While in skip mode, the N-

channel is kept off to optimize efficiency by reducing

gate charge dissipation. VOUT is regulated by skipping

switching cycles that turn on the P-channel.

To begin analyzing MIC2177 skip-mode operation,

assume the skip-mode comparator output is high and

the latch output has been reset to a logic 1. This turns on

the P-channel and causes IL1 to increase linearly until it

reaches a current limit of 600mA. When I L1 reaches this

value, the current limit comparator sets the RS latch

output to logic 0, turning off the P-channel. The output

switch voltage (VSW) then swings from VIN to 0.4V below

ground, and IL1 flows through the Schottky diode. L1

discharges its energy to the output and IL1 de-creases to

zero. When IL1 = 0, VSW swings from –0.4V to VOUT, and

this triggers a one-shot that resets the RS latch.

Resetting the RS latch turns on the P-channel, which

begins another switching cycle.

The skip-mode comparator regulates VOUT by controlling

when the MIC2177 skips cycles. It compares VFB to VREF

and has 10mV of hysteresis to prevent oscillations in the

control loop. When VFB is less than VREF – 5mV, the

comparator output is logic 1, allowing the P-channel to

turn on. Conversely, when VFB is greater than VREF +

5mV, the P-channel is turned off.

Note that this is a self-oscillating topology which explains

why the switching frequency and duty cycle are a

function of VIN, VOUT, and the value of L1. It has the

unique feature (for a pulse-skipping regulator) of

supplying the same value of maximum load current for

any value of VIN, VOUT, or L1. This allows the MIC2177 to

always supply up to 300mA of load current (ILOAD) when

operating in skip mode.

Changing from PWM to Skip Mode

Refer to “Block Diagram” for circuits described in the

following sections.

The MIC2177 automatically changes from PWM to skip

mode operation when ILOAD drops below a minimum

value. IMIN is determined indirectly by detecting when the

peak inductor current (IL(peak)) is less than 420mA. This is

done by the minimum current comparator which detects

if the output P-Channel current equals 420mA during

each switching cycle. If it does not, the PWM/skip-mode

select logic places the MIC2177 into skip-mode

operation.

The value of IMIN that corresponds to IL1(peak) = 420mA is

given by the following equation:

∆I

420mA

MIN

−

Where:

∆IL1 = inductor ripple current

This equation shows IMIN varies as a function of ∆IL.

Therefore, the user must select an inductor value that

results in IMIN = 200mA when IL(peak) = 420mA. The

formulas for calculating the correct inductor value are

MIC2177BWM Datasheet(PDF) 9 Page - Micrel Semiconductor

MIC2177BWM Datasheet(HTML) 9 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Datasheet Download

Link URL