L5981 STMicroelectronics, L5981 Datasheet - Page 20

• 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

Application information
20/37
Equation 18
where K is the feed forward constant and 1 / K is equals to 9.
3.
Equation 19
4.
Equation 20
5.
Equation 21
The suggested maximum system bandwidth is equals to the switching frequency divided by
3.5 (F
For example with V
the type III compensation network is:
In
56 kHz and the phase margin is 53 °.
R
1
Figure 12
=
Calculate C
Calculate C
Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
4.99kΩ
SW
/ 3.5), anyway lower than 100 kHz if the F
,
is shown the module and phase of the open loop gain. The bandwidth is about
R
4
5
2
by placing the zero at 50 % of the output filter double pole frequency (f
by placing the second pole at four times the system bandwidth (BW):
=
OUT
1.1kΩ
= 3.3 V, V
R
,
3
R
=
3
-------------------------- -
4 BW
----------------- 1
Doc ID 13004 Rev 6
=
⋅
f
C
LC
110Ω
IN
5
R
1
= 12 V, I
=
–
R
------------------------------------------------------------- -
2π R
C
,
4
4
,
R
⋅
=
=
4
BW K
----------------- - R
4
=
-------------------------- -
π R
O
⋅
f
⋅
5.6kΩ
= 1 A, L =33 μH, C
LC
C
⋅
C
C
4
1
4
4
⋅
3
⋅
4 BW
⋅
=
f
SW
LC
,
⋅
---------------------------------------- -
2π R
1
C
is set higher than 500 kHz.
⋅
3
–
=
3
1
1
4.7nF
⋅
4 BW
⋅
,
OUT
C
= 22 μF, ESR < 1 mΩ,
4
=
10nF
,
C
5
=
100pF
L5981
LC
):