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MIC5031
Applications Information
Power-Up Sequence
The supply voltage (V
DD
) must be applied to VDD before EN
is asserted. If EN is not required for the application, an RC
network must be used to delay the voltage rise applied to EN
with respect to VDD. See Figure 1.
Micrel
MIC5031
6
August 1999
On
Off
FLG
RV
G
VDD
EN
CTL
+4.5V to +30V
MIC5031
CB
CP1+
S
CP1–
RI
CP2+
CS
CP2–
DLY
GND
15μF
12k*
0.1μF
0.01μF
0.01μF
7
4
3
10
8
11
9
5
2
6
15
13
12
16
14
1
IRF540
0.1μF
M
2N5822
100μF
100nF
0.01μF
10k
Figure 1. Enable Application
Refer to “Typical Application” for controlling EN from open-
collector or open-drain logic. The 10k resistor and 0.01
μ
F
capacitor connected to VDD, GND, and EN keep EN low
during power up before the open-collector or open-drain logic
becomes active.
The 10k resistor and 0.01
μ
F capacitor can be omitted if EN
is held low by the external logic until VDD is powered.
Overcurrent Detection
Using the MOSFET manufacturer’s data and the maximum
allowable load current, determine the maximum drain-to-
source voltage drop, V
DS
, that will occur across the external
MOSFET in normal operation. This will also be the reference
voltage and the overcurrent trip voltage, V
R1
.
V
R1
= maximum R
DS(on)
×
maximum load current
Supply
1.23V
Bandgap
Reference
RV
RI
External
N-Channel
MOSFET
L
V
DS
V
R1
I
R2
R1
R2
1.23V
I
R2
Figure 2. Resistor Calculations
Reference Current Resistor
Resistor R2 sets the reference current. For most applica-
tions, a reference current of 100
μ
A is suggested.
R2
R1
I
R2
=
where:
R2 = reference current resistor (
)
I
R2
= reference current (A) [R2 = 12k
for
approximately 100
μ
A]
Reference Voltage Resistor
The reference voltage resistor value is calculated from the
reference current and the reference voltage (overcurrent
drop voltage).
R1
V
I
R2
R1
=
where:
R1 = reference voltage resistor (
)
V
R1
= reference voltage (V) [see above]
I
R2
= reference current (A) [see above]
Overcurrent Delay Capacitor
For lamp switching applications, the delay capacitor (C
DLY
)
may be as high as several microfarads. Lamps often have an
inrush current of 10
×
their steady-state operating current. In
PWM applications, pay attention to the input frequency vs.
the overcurrent delay. They can conflict with each other if not
properly planned.