Rainbow Electronics MAX5929 - Manual (Page 19)

Startup Sequence
There are two ways of completing the startup
sequence. Case A describes a startup sequence that
slowly turns on the MOSFETs by limiting the gate
charge. Case B uses the current-limiting feature and
turns on the MOSFETs as fast as possible while still
preventing a high inrush current. The output voltage
ramp-up time (t
ON
) is determined by the longer of the
two timings, case A and case B. Set the startup timer
(t
START ) to be longer than t ON to guarantee enough
time for the output voltage to settle.
Case A: Slow Turn-On (Without Current Limit)
There are two ways to turn on the MOSFETs without
reaching the fast-comparator current limit:
• If the board capacitance (C BOARD ) is small, the
inrush current is low.
• If the gate capacitance is high, the MOSFETs turn
on slowly.
In both cases, the turn-on time is determined only by
the charge required to enhance the MOSFET. The
small 100µA gate-charging current effectively limits
the output voltage dV/dt. Connecting an external
capacitor between GATE and GND extends the turn-
on time. The time required to charge/discharge a
MOSFET is as follows:
where:
C
GATE
is the external gate to ground capacitance
(Figure 9),
∆V
GATE is the change in gate charge,
Q GATE is the MOSFET total gate charge,
I
GATE
is the gate-charging/discharging current.
In this case, the inrush current depends on the MOSFET
gate-to-drain capacitance (C
rss
) plus any additional
capacitance from GATE to GND (C GATE ), and on any
load current (I LOAD ) present during the startup period.
Example: Charging and discharging times using the
Fairchild FDB7030L MOSFET
If V IN1 = 5V then GATE1 charges up to 10.4V (V IN1 +
V
DRIVE
), therefore ∆V
GATE = 10.4V. The manufacturer’s
data sheet specifies that the FDB7030L has approxi-
mately 60nC of gate charge and C
rss = 600pF. The
MAX5927/MAX5929 have a 100µA gate-charging cur-
rent and a 3mA/50mA normal/strong discharging cur-
rent. C
BOARD = 6µF and the load does not draw any
current during the startup period. With no gate capaci-
tor, the inrush current, charge, and discharge times are:
I
F
pF
A A
I
V nC
A
ms
t
V nC
mA
ms
t
V nC
mA
s
INRUSH
CHARGE
DISCHARGE
DISCHARGE STRONG
=
µ
+
Ч + =
=
Ч +
µ
=
=
Ч +
=
=
Ч +
=
6
600 0
100 0 1
0 10 4 60
100
0 6
0 10 4 60
3
0 02
0 10 4 60
50
1 2
.
.
.
.
.
.
( )
µ
µ
I
C
C C
I I
INRUSH
BOARD
rss GATE
GATE LOAD
=
+
× +
t
C V Q
I
GATE GATE GATE
GATE
=
× + ∆
MAX5927/MAX5929
Low-Voltage, Quad, Hot-Swap
Controllers/Power Sequencers
______________________________________________________________________________________ 19
Figure 10. Adjustable Undervoltage Lockout
GATE_ SENSE_
V TURN-ON -
(R 2 x R 1 ) V ON , TH
R 2
ON_
V IN
IN_
R 1
R 2
MAX5927
MAX5929
Figure 9. Operating with an External Gate Capacitor
GATE_ SENSE_
GND
ON_
R SENSE_
V OUT_
C GATE
C BOARD
V IN_
IN_
R PULLUP
STAT_
MAX5927
MAX5929