iru3065 International Rectifier Corp., iru3065 Datasheet - Page 5

• Manufacturer Part Number: iru3065
• Description: Positive To Negative Dc To Dc Controller
• Manufacturer: International Rectifier Corp.

APPLICATION EXAMPLE
Design Example
The following design example is for the evaluation board
application for IRU3065. The schematic is shown in fig-
ure 1:
Voltage Sensing Resistor
The output voltage is determined by the two voltage sens-
ing resistors R2 and R3:
If R3 is chosen as 10K, Then R2 is given by:
Current Sensing Resistor R
In order to select R
has to be determined. Considering the switching losses,
for conservative, the critical current should select to be
slightly greater than the nominal output current.
Where 1.5 is the coefficient to take the efficiency into
account.
According to equation (1), the current I
The current sensing resistance is calculated as:
Select R
From equation (3), the modified inductor peak current
is:
Where:
V
V
I
f
f
V
V
Vcc = 5V
VI
V
R2 = -
Select:
I
I
R
R
I
OCP
OCP
OUT
S(MAX)
S(MAX)
PEAK
IN
OUT(NOM)
D
D
OUT(NOM)
S
S
SEN(TH)
= Diode Forward Voltage
= 0.5V
=
=
= 5V
= 200mA
= 200mA 1.5 = 300mA
=
S
=
1
2
1
2
= Maximum Frequency
= 1.2MHz
= 0.1
1
2
V
VI
=145mV 150mV
OUT(NOM)
V
SEN(TH)
= -5V
= -
R
0.15
0.15
I
REF
0.3
OCP
0.15
S
R
R3
R2
S
= 1.5A
S
5 - (-5) + 0.5
V
, the desired critical current I
V
IN
R3 = -
V
IN
- V
REF
- V
5
OUT(NOM)
OUT(NOM)
V
-5V
5V
S
V
IN
IN
+ V
y 0.12
+ V
10K = 10K
OCP
D
D
= 300mA
is given by:
www.irf.com
OCP
The modified current I
Output Inductor L
The inductance is chosen by equation (2):
The maximum inductor current is: I
The maximum average inductor current equals
MOSFET Selection
A P-channel MOSFET is required. The peak current in
this case is equal to I
IRLML5203, from international Rectifier with I
BVDSS=30V, is a good choice.
Input Capacitor
An input capacitor will help to minimize the induced
ripple on the +5V supply. A 1 F to 10 F X7R ceramic
capacitor is recommended.
Output Capacitor
An output capacitor is required to store energy from
transfer to the output inductor. Its capacitance and ESR
have a great impact on output voltage ripple. A 10 F to
22 F X7R Tantolum or ceramic capacitor is recom-
mended.
Output Diode
The average diode current equals output current. In
this case, select the diode average current larger than
300mA. The lowest block voltage is V
case, It is 10V. In order to reduce the switching losses,
the Schottky diode is recommended. The diode 10BQ015
from International Rectifier with I
a good choice.
Other Components
In order to speed up the turn off of P-channel MOSFET,
a fast diode 1N4148 or a 100ohm resistor and 100pF
capacitor is connected to the pin V
I
AVG
I
I
I
L
L
Select L = 1.2 H
AVG
OCP
OCP
=(145mV+50mV)/0.1ohm/2=1A
=(VI
1.5A
=
=
5
1
2
1
2
SENTH_MAX
V
OUT(NOM)
V
(5 - (-5) + 0.5) 1.2MHz
V
IN
5 + 0.5 - (-5)
IN
+ V
(V
+VI
(V
) f
5
D
-(-5 - 0.5)
OCP
IN
D
SENTH_MIN
- V
V
+V
- V
S(MAX)
IN
is:
PEAK
OUT(NOM)
D
OUT(NOM)
IRU3065(PbF)
-
=1.5A. The MOSFET
)/Rs/2
I
PEAK
D
DD
1.5A = 357mA
)
=1A and V
PEAK
and V
IN
+(-V
0.15
= 1.5A
R
GATE
= 1.45 H
S
OUT
BR
D
as shown
=3A and
). In this
=15V is
5