MP3430 — 90V STEP-UP CONVERTER WITH APD CURRENT MONITOR

MP3430 Rev 1.11

www.MonolithicPower.com

11

4/25/2013

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energy in the SW MOSFET capacitor followed by

LC ringing.

There is a “reverse current” – current going from

Negative Going

Inductor Current

Due to the applied high-output voltage on the

coupling of the MP3430 FET, a significant

reverse current flows through the inductor during

This negative inductor current turns the FET

voltage applied by the conducting body diode to

the SW node) causes the inductor current to

ramp up from the maximum negative going

current to about 60% of that magnitude in the

positive direction—where the positive current

Ringing current occurs after the current turns off

time for the current to turn off the FET body diode

following equations:

MAX, REVERSE

OUT

40pF

IV

L



MAX,REVERSE

Re verseCurrent

IN,MIN

1.6 L I

t

V1







2

OUT

1

IN

2V

K

D2.2

1

1

4V



















1IN

2

OUT

IN

DV

D

VV







31

2

D1 D D

 

S

3

Re verseCurrent

Dt

t

 

Where,

SOUT

OUT

2L f

I

K

V1000

 





,

Staying in Discontinuous Conduction Mode

(DCM)

The system must operate in discontinuous

conduction mode (DCM) to maintain stability due

to the high conversion ratio from VIN to VOUT. A

boost converter has a right-hand zero that can

cause system instability if that zero moves into

the

system’s

operational-frequency

range.

Furthermore the right hand zero moves into lower

frequencies—where the system operates—as the

conversion ratio increases. This right-hand zero

does not exist when operating in DCM

Stability therefore requires that the system

operates in DCM under all conditions. To this end,

a dimensionless parameter called K measures a

system’s tendency to operate in DCM mode. The

(continuous conduction mode) system boundary.

operation.



IN

IN

CRIT

CCM

CCM

OUT

OUT

2

V

2

V

KD

D

1

V

V











 





SOUT

OUT

2L f

I

K

V1000

 





CRIT

OUT

SOUT

K

V

1000

L

2f I









L: µH

There is a size limit to the inductor that can

cause the system to enter CCM mode and risk

instability.