MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 Rev. 1.0

www.MonolithicPower.com

9

11/5/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

comparator is first blanked till the end of the turn-

on transition to avoid noise issues. The

comparator then compares the power switch

current with the COMP voltage. When thesensed

current is higher than the COMP voltage, the

comparator output is low, turning off the power

MOSFET. The cycle-by-cycle maximum current

of the internal power MOSFET is internally

limited.

Short Circuit Protection

When the output is shorted to the ground, the

switching frequency is folded back and the

current limit is reduced to lower the short circuit

current. When the voltage of FB is at zero, the

current limit is reduced to about 50% of its full

current limit. When FB voltage is higher than

0.4V, current limit reaches 100%.

In short circuit FB voltage is low, the SS is pulled

down by FB and SS is about 100mV above FB.

In case the short circuit is removed, the output

voltage will recover at the SS pace. When FB is

high enough, the frequency and current limit

return to normal values.

Startup and Shutdown

If both VIN and EN are higher than their

appropriate thresholds, the chip starts. The

reference block starts first, generating stable

reference voltage and currents, and then the

internal regulator is enabled. The regulator

provides stable supply for the remaining circuits.

While the internal supply rail is up, an internal

timer holds the power MOSFET OFF for about

50µs to blank the startup glitches. When the

external soft-start block is enabled, it first holds

its SS output low to ensure the remaining circuits

are ready and then slowly ramps up at a rate

capacitor.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In the shutdown

procedure, power MOSFET is turned off first to

avoid any fault triggering. The COMP voltage and

the internal supply rail are then pulled down.

Programmable Oscillator

The MP4561 oscillating frequency is set by an

from:

FREQ

S

100000

R(k )

5

f(k Hz)

Ω =−

Soft-Start

The soft-start is implemented to prevent the

converter output voltage from overshooting

during startup and short circuit recovery phases.

applied on the error amplifier in parallel with the

internal reference voltage REF. The error

amplifier is controlled by VSS or REF whichever

the controller tries to regulate FB from zero to

There is a 5μA current source pulling up SS pin.

Given the soft-start capacitor CSS, the soft-start

time is about the time CSS voltage changes 0.8V.

So the soft-start time can be computed as:

SS

SS

C0.8 V

t

5μA

×

=

Soft-start Time vs.

Soft-start Capacitance

0

5

10

15

20

25

30

35

5

55

105

155

205

SS CAPACITANCE (nF)

Figure 2—Recommend SS time vs.

SS Capacitance

Figure 2 shows the soft-start time with different

external soft-start capacitance. The typical soft-

start capacitance is recommended from 5.6nF to

220nF.