mic9130 Micrel Semiconductor, mic9130 Datasheet - Page 15

• Manufacturer Part Number: mic9130
• Description: High-voltage, High-speed Telecom Dc-to-dc Controller
• Manufacturer: Micrel Semiconductor

series resistance is 10K, for a 500kHz switcher, the maximum
series resistance is 2K.
Sensing Current with a Resistor
The fast transition times of the current signal prohibit the use
of inductive resistors. Standard wire wound power resistors
will not work. Carbon composition or metal film resistors or
low inductance power resistors may be used. The overcurrent
range of the power supply and component tolerances must be
considered when selecting the current sense resistor value.
The power supply specification may call for an overcurrent
limit, which must be accounted for when selecting the cur-
rent sense resistor value. The relationship between the peak
primary current and the current sense resistor is:
where: Ip is the current in the sense resistor
The current sense resistor must not be too small or the cur-
rent sense signal will be susceptible to noise. If noise is a
problem, the current signal level should be increased.
An example is illustrated below.
The maximum peak current, I
and minimum input voltage
The maximum rms current, I
The desired current sense signal amplitude is 500mV at 1A
output current.
The current sense resistor value and power dissipation is:
A 0.5Ω, non inductive resistor with at least a 1/2W rating
should be selected.
The series resistor is calculated to allow the 500mV-peak
signal to reach 0.82V.
The next lower value of 10kΩ is selected.
The bandwidth of the 10K resistor and the 25pF input capaci-
tance is calculated. The resistor value must be lowered if the
bandwidth is too low for the switching frequency.
The maximum switching frequency of this power supply
should be approximately six times less than the BW to pre-
vent current waveform distortion and excessive delays in
the current loop. This limits the switching frequency to the
range of 100kHz.
April 2005
MIC9130
R
I
(40µA)
R
the current sense resistor.
P
R
BW
V
R
ISNS
DISS
ISNS
ISENSE
f
SENSE
f
is the series resistor between the ISNS pin and
V
ISNS
=
is the current sourced from the ISNS pin
=
=
2
=
I
I
P
RMS
× ×
−
is the current sense resistance
V
I
×
SENSE
(
π π
SENSE
I
I
R
ISNS
2
P
ISENSE
×
×
10
R
R
1
SENSE
ISENSE
k
=
×
0.5
+
25
RMS
1
PMAX
I
ISNS
=
pF
)
=
=
=0.65A
0 65
0.5
.
×
= 1A at 120% overcurrent
0 82
=
.
R
Ω
630
2
f
40
×
− ×
kHz
0 5
(
µ
1 0 5
.
A
=
.
0 21
.
)
=
W
10 25
.
k
Ω
15
Sensing Current with a Current Sense Transformer
At higher power levels, the power dissipation in a current sense
resistor is excessive. A current sense transformer can be
used to sense the current while minimizing power dissipation.
See Figure 11. The schematic shows the circuitry necessary
when using a current sense transformer. The resistor, R1,
provides a path to reset the current sense transformer. The
resistor, R2, converts the scaled down current to a voltage,
which is sent to the ISNS pin.
The voltage at the ISNS pin is calculated by:
where: I
Current Transformer example:
The maximum peak current, I
rent and minimum input voltage
The maximum rms current, I
The full 0.82V peak signal a the ISNS input can be used
since very little power is dissipation in the secondary
side sense resistor. The maximum peak to peak volt-
age at the sense pin (pin 14) is 0.82V at the 5A maximum
output current.
The current sense resistor value and power dissipation
is:
R
P
V
transformer
R2 is the current sense resistance at the
secondary of the current sense transformer
N is the turns ratio of the current sense
transformer (N=Nsec/Npri)
I
(40µA)
R
the current sense resistor.
P
ISNS
DISS
MIC9130
ISNS
f
2
is the current in the primary of the current sense
is the series resistor between the ISNS pin and
=
=
V
is the current sourced from the ISNS pin
=



SENSE
(pin 14)
(pin 16)
I
I
N
P
PRMS
OUT
N
I
I
SNS
×
P
R

 ×

×
2
2
N
+
Rf
Figure 11
R
=
I
ISNS
2
0 82 100
=
.
R2



×
RMS
PMAX
3 25
100
R
V
5
×
.
IN
f
= 3.25A

 ×

2
= 5A at 120% overcur-
R1
=
16 4 17 4
16 4
.
.
=
Ω
Current Sense
Transformer
I
PRI
M9999-040805
.
Micrel, Inc.
mW