MP2174C

– 5.5V, 4A, 1.1MHZ, SYNCHRONOUS STEP-DOWN SWITCHER

MP2174C Rev. 1.0

www.MonolithicPower.com

12

8/7/2019

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© 2019 MPS. All Rights Reserved.

OPERATION

The MP2174C uses constant-on-time control

with input voltage feed forward to stabilize the

switching frequency over the full input range.

Constant-On-Time Control

Compare

to fixed-frequency

PWM control,

constant-on-time control offers the advantage of

a simpler control loop and faster transient

response. By using input voltage feed forward,

the MP2174C maintains a nearly constant

switching frequency across the input and output

voltage ranges. The on-time of the switching

pulse can be estimated with Equation (1):







OUT

ON

IN

V

t

0.909 s

V

(1)

To prevent inductor current runaway during

load transient, the MP2174C limits the minimum

off time to 100ns. This minimum off time limit

does not affect the operation of the MP2174C in

steady state in any way.

Forced PWM Operation

The MP2174C works in continuous conduction

mode (CCM) to achieve a smaller VO ripple,

load regulation, and load transient across the

full load range.

Enable

When the input voltage exceeds the under-

voltage lockout (UVLO) threshold (typically

2.5V), the MP2174C can be enabled by pulling

the EN pin above 0.65V (typical). Leaving EN

below 0.55V (typical) or floating it disables the

MP2174C. There is an internal 1M

Ω resistor

from the EN pin to ground.

When EN is pulled down, there is a discharge

path for output. All processes, from EN turning

off to the output dropping to 0V, take less than

0.8ms.

Soft Start/Stop

The MP2174C has a built-in soft start that

ramps up the output voltage in a controlled slew

rate, avoiding overshoot at start-up. The soft-

start time is typically about 0.5ms.

When disabled, the MP2174C ramps down the

internal reference and allows the load to linearly

discharge the output. The soft-stop time is less

than 0.3ms.

Power Good Indicator

The MP2174C has an open-drain output. The

PG pin requires an external pull-up resistor

(100k

Ω to 500kΩ) for the power good indicator.

When the FB pin is within ±10% of the

regulation voltage (i.e. 0.6V), the PG pin is

pulled up. If the FB voltage is out of the ±10%

window, PG is pulled down to ground by an

internal

MOSFET.

The

MOSFET

has

a

maximum RDS(ON) of less than 100Ω.

Current Limit

The MP2174C has a typical 6.5A current limit

for the HS-FET, and a 4.5A current limit for the

LS-FET. Once the high-side switch hits current

limit, the MP2174C turns off the HS-FET and

turns on the LS-FET to reduce the inductor

current. When the inductor current drops to the

valley current limit, the LS-FET turns off and the

HS-FET turns on. If the HS-FET reaches the

peak current limit and the LS-FET reaches the

valley current limit every cycle for 150µs, the

MP2174C remains in hiccup mode until the

current decreases. This prevents the inductor

current from continuing to build up, which could

result in damage to the components.

Short Circuit and Recovery

If the output voltage of the buck converter is

shorted to GND, the current limit is triggered. If

the current limit is triggered every cycle for

150µs, the MP2174C enters hiccup mode. It

disables the output power stage, discharges the

soft-start capacitor, and then automatically

retries soft start. If the short-circuit condition still

remains after soft start ends, the MP2174C

repeats this operation cycle until the short

condition is removed and the output rises back

to the regulation level.