MP2176 – 6V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP2176 Rev. 1.0

www.MonolithicPower.com

13

4/17/2018

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OPERATION

Pulse-Width Modulation (PWM) Operation

The MP2176 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

Constant-on-time (COT) control is employed to

provide fast transient response and ease loop

stabilization. At the beginning of each cycle, the

high-side MOSFET (HS-FET) is turned on when

the

feedback

voltage

(VFB)

is

below

the

reference voltage (VREF), which indicates an

insufficient output voltage. The on period is

determined by the input voltage, and the

frequency-set

resistor

is

determined

with

Equation (1):

FREQ

ON

IN

4.8 R

(k )

t

(ns)

V (V) 0.49







(1)

After the on period elapses, the HS-FET is

turned off. The HS-FET is turned on again

when VFB drops below VREF. By repeating

operation in this way, the converter regulates

the output voltage. The integrated low-side

MOSFET (LS-FET) is turned on when the HS-

FET is in its off state to minimize conduction

loss. There is a dead short between the input

and GND if both the HS-FET and LS-FET are

turned on at the same time. This is called

shoot-through. To avoid shoot-through, a dead

time (DT) is generated internally between the

HS-FET off and LS-FET on period or the LS-

FET off and HS-FET on period.

Figure 2: PWM Operation

The MP2176 always operates in continuous

conduction mode (CCM), which means that the

inductor current can go negative in light-load

operation (see Figure 2). When VFB is below

VREF, the HS-FET is turned on for a fixed

interval determined by the one-shot on-timer.

When the HS-FET is turned off, the LS-

MOSFET is turned on until the next period

begins. When the MP2176 operates in CCM,

the switching frequency is fairly constant. This

is called pulse-width modulation (PWM) mode.

Switching Frequency

Selecting the switching frequency is a trade-off

between efficiency and component size. Low-

frequency operation increases efficiency by

reducing

MOSFET

switching

losses,

but

requires a larger inductance and capacitance to

maintain a low output voltage ripple.

For the MP2176, the on time can be set using

FREQ. In this way, the frequency is set in

steady-state operation in CCM.

Adaptive COT control is used in the MP2176,

and there is no dedicated oscillator in the IC.

Connect FREQ to IN through a resistor (RFREQ),

and the input voltage is forward fed to the one-

shot on-time timer through RFREQ. In steady-

state operation at CCM, the duty ratio is kept as

VOUT/VIN. Therefore, the switching frequency is

fairly constant over the input voltage range. The

switching frequency can be set with Equation

(2):

6

SW

FREQ

IN

DELAY

IN

OUT

10

f

(kHz)

4.8 R

(k )

V (V)

t

(ns)

V (V) 0.49

V

(V)









(2)

Where tDELAY is the comparator delay (about

40ns).

Generally, the MP2176 is set for 300kHz -

1MHz applications and is optimized to operate

at

a

high

switching

frequency

with

high

efficiency. With a high switching frequency,

small LC filter components can be used to save

system PCB space.

Jitter and FB Ramp Slope

Figure 3 shows jitter occurring in PWM mode.

When there is noise in the VFB downward slope,

the on time of the HS-FET deviates from its

intended location and produces jitter. Note that

t

here is a relationship between a system’s

stability and the steepness of the VFB ripple’s

downward slope. The slope steepness of the

VFB ripple dominates in noise immunity. The

magnitude of the VFB ripple does affect noise

immunity directly.