L5981 STMicroelectronics, L5981 Datasheet - Page 15

• Manufacturer Part Number: L5981
• Description: IC REG SW STEP DOWN 1A 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5981

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 1A
• Frequency - Switching: 250kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• Output Voltage: 3 V
• Output Current: 1 A
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 220 KHz to 275 KHz
• Operating Temperature Range: - 40 C to + 150 C
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• Low Dropout Operation: 100 % duty cycle
• For Use With: 497-6385 - BOARD EVAL FOR L5981
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

L5981
5.2
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value in order to have the expected current ripple has to be selected.
The rule to fix the current ripple value is to have a ripple at 20 % - 40 % of the output current.
In the continuos current mode (CCM), the inductance value can be calculated by the
following equation:
Equation 6
Where T
time of the external diode (in CCM, F
fixed Vout, is obtained at maximum T
to calculate minimum duty). So fixing ΔI
minimum inductance value can be calculated:
Equation 7
where F
For example for V
inductance value to have ΔI
The peak current through the inductor is given by:
Equation 8
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
In the table below some inductor part numbers are listed.
Table 7.
Manufacturer
Coiltronics
SUMIDA
Coilcraft
SW
ON
Wurth
is the conduction time of the internal high side switch and T
is the switching frequency, 1/(T
Inductors
OUT
= 3.3 V, V
ΔI
L
L
CDRH105RNP
CDR6D28MN
= 30 % of I
=
MSS1038
LPS6235
Doc ID 13004 Rev 6
DRQ73
Series
V
----------------------------- - T
IN
PD M
L
LD2
IN
MIN
= 12 V, I
–
L
OFF
V
SW
=
OUT
I
L PK
L
,
V
--------------------------- -
O
= 1 / (T
= 20 % to 40 % of the maximum output current, the
that is at minimum duty cycle (see previous section
OUT
ΔI
is about 31 μH.
⋅
O
ON
MAX
=
= 1 A and F
ON
+
+ T
I
O
ON
V
=
Inductor value (μH)
+
F
OFF
⋅
ΔI
------- -
V
--------------------------- - T
+ T
2
1 D
---------------------- -
OUT
L
–
).
F
OFF
10 to 18
22 to 47
10 to 18
10 to 22
27 to 47
10 to 22
27 to 56
SW
L
+
MIN
SW
)). The maximum current ripple, at
V
F
= 250 kHz the minimum
⋅
OFF
Application information
Saturation current (A)
OFF
is the conduction
1.67 to 2.47
1.64 to 2.1
1.65 to 2.5
1.7 to 2.2
1.9 to 2.9
1.8 to 2.4
1.9 to 2.7
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