MP3430 Datasheet, PDF (11/15 Pages) Monolithic Power Systems – 90V Step-Up Converter with APD Current Monitor

MP3430 — 90V STEP-UP CONVERTER WITH APD CURRENT MONITOR
D₃×t_S: the L current reverses due to
energy in the SW MOSFET capacitor followed by
LC ringing.
There is a “reverse current” – current going from
the SW node back into V_IN – during D₃.
V_SW
Negative Going
Inductor Current
Due to the applied high-output voltage on the
switch node combined with the C_DS capacitive
coupling of the MP3430 FET, a significant
reverse current flows through the inductor during
the D₃ period.
The energy stored in C_DS transfers to the inductor.
This negative inductor current turns the FET
body diode on. V_IN (combined with the negative
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
goes from V_IN to the SW node, and the negative
current feeds back into V_IN through the inductor.
Ringing current occurs after the current turns off
the body diode. D₃ is always greater than the
time for the current to turn off the FET body diode
and to start ringing. Determine D₃ as per the
following equations:
IMAX, REVERSE  VOUT 
40pF
L
t_{Re verseCurrent}

1.6
 L  I_MAX,REVERSE
V_IN,MIN  1
D₁  2.2
K
4


2V_OUT
V_IN
 1^{2}


 1

D2

D1  VIN
VOUT  VIN
D3  1 D1  D2
D3  t_S  t_{Re verseCurrent}
Where,
K

2  L  fS  IOUT
VOUT 1000
,
V_OUT: V, L: µH, f_S: MHz, I_OUT: mA
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
other parameter is K_CRIT which is the DCM, CCM
(continuous conduction mode) system boundary.
If K<K_CRIT, then the system is in DCM mode
operation.
2
  ^{  KCRIT  DCCM }
DCCM
2

_1

VIN
VOUT




VIN
VOUT
K

2  L  fS  IOUT
VOUT  1000
DCM Mode: K < K_CRIT:
L

KCRIT  VOUT 1000
2  fS IOUT
V_IN, V_OUT: V
L: µH
f_S: MHz
I_OUT: mA
There is a size limit to the inductor that can
cause the system to enter CCM mode and risk
instability.
MP3430 Rev 1.11
www.MonolithicPower.com
11
4/25/2013
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.