MAX5930A/MAX5931A/MAX5931B
Setting the Startup Period, R
TIM
The startup period (t
START
) is adjustable from 0.4ms to
50ms. The adjustable startup period feature allows sys-
tems to be customized for MOSFET gate capacitance
and board capacitance (C
BOARD
). The startup period is
adjusted with a resistor connected from TIM to GND
(R
TIM
). R
TIM
must be between 4kΩ and 500kΩ. The
startup period has a default value of 9ms when TIM is left
unconnected. Calculate R
TIM
with the following equation:
where t
START
is the desired startup period.
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),
ΔV
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 ΔV
GATE
= 10.4V. The manufacturer’s
data sheet specifies that the FDB7030L has approxi-
mately 60nC of gate charge and C
RSS
= 600pF. The
MAX5930A/MAX5931A/MAX5931B have a 100µA gate
charging current and a 3mA/50mA normal/strong dis-
charging current. C
BOARD
= 6µF and the load does not
draw any current during the startup period. With no gate
capacitor, the inrush current, charge, and discharge
times are:
IRC, Inc. 361-992-7900 www.irctt.com
