MP2918

—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2918 Rev. 1.02

www.MonolithicPower.com

19

5/31/2017

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Error Amplifier (EA)

internal 0.8V reference and outputs a current

proportional to the difference between the two

input voltages. This output current is used to

charge or discharge the external compensation

the power MOSFET current. Adjusting the

compensation network from COMP to SGND

optimizes the control loop for good stability or

fast transient response.

Current Limit Function

The MP2918 has three fixed current limit

options: 25mV, when ILIM is connected to

SGND; 50mV, when ILIM is connected to VCC1;

and 75mV, when ILIM is floating.

When the peak value of the inductor current

exceeds the set current-limit threshold, the

output voltage begins dropping until FB is

37.5% below the reference. The MP2918 enters

hiccup mode to restart the part periodically. The

frequency is lowered when FB is below 0.4V.

This protection mode is especially useful when

the output is dead-shorted to ground. The

average short-circuit current is reduced greatly

to alleviate thermal issues. The MP2918 exits

hiccup mode once the over-current condition is

removed.

Low Dropout Operation

In low dropout mode, the MP2918 is designed

to operate in high-side fully on mode for as long

as the voltage difference across BST - SW is

greater than 3.05V, improving dropout. When

the voltage from BST to SW drops below 3.05V,

a UVLO circuit turns off the HS-FET. At the

same time, the low-side MOSFET (LS-FET)

turns on to refresh the charge on the BST

capacitor. After the BST capacitor voltage is re-

charged, the HS-FET turns on again to regulate

the output. Since the supply current sourced

from the BST capacitor is low, the HS-FET can

remain on for more switching cycles than are

required

to

refresh

the

BST

capacitor,

increasing the effective duty cycle of the

switching regulator. Low dropout operation

makes the MP2918 suitable for automotive

cold-crank applications.

Power Good (PG) Function

The MP2918 includes an open-drain power

good (PG) output that indicates whether the

regulator’s output is within ±10% of its nominal

value. When the output voltage falls outside of

this range, the PG output is pulled low. PG

should be connected to a voltage source no

more than 5V through a resistor (e.g.: 100k

Ω).

The PG rising delay time is 37µs, and the PG

falling delay time is 28µs.

Soft Start (SS)

Soft start (SS) is implemented to prevent the

converter output voltage from overshooting

during start-up. When the chip starts up, the

internal circuitry generates a soft-start voltage

that ramps up from 0V to 1.2V. When it is lower

than REF, SS overrides REF, so the error

amplifier uses SS as the reference. When SS is

higher than REF, REF regains control.

An external capacitor connected from SS to

SGND is charged from an internal 4

μA current

source, producing a ramped voltage. The soft-

capacitor and can be calculated by Equation (3):

 

 

 

 

μA

I

V

V

nF

C

ms

t

SS

REF

SS

SS





(3)

the 4μA SS charge current. There is no internal

SS

capacitor.

SS

is

reset

when

a

fault

protection other than over-voltage protection

(OVP) occurs.

Output Over-Voltage Protection (OVP)

the MP2918 enters discharge mode. The HS-

FET turns off, and the LS-FET turns on. The

LS-FET remains on until the reverse current

limit is triggered. The LS-FET then turns off,

and the inductor current increases to 0. The LS-

FET is turned on again after ZCD is triggered.

The MP2918 works in discharge mode until the

over-voltage condition is cleared.