L5981 STMicroelectronics, L5981 Datasheet - Page 23

• 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
Figure 14. Open loop gain: module Bode diagram
The guidelines for positioning the poles and the zeroes and for calculating the component
values can be summarized as follow:
1.
2.
Equation 22
Where f
Equation 23
and Vs is the saw-tooth amplitude. The voltage feed forward keeps the ratio Vs/Vin constant.
3.
Equation 24
4.
Choose a value for R
C5 not comparable with parasitic capacitance of the board.
Choose a gain (R
Calculate C
Then calculate C
(BW):
ESR
is the ESR zero:
4
by placing the zero one decade below the output filter double pole:
3
4
in order to place the second pole at four times the system bandwidth
/R
1
1
) in order to have the required bandwidth (BW), that means:
, usually between 1 k and 5 k, in order to have values of C4 and
Doc ID 13004 Rev 6
R
4
f
ESR
=
C
⎛
⎝
f
----------- -
4
ESR
f
=
LC
=
------------------------------------------- -
2π ESR C
⎞
⎠
------------------------------ -
2π R
2
⋅
⋅
⋅
----------- -
f
BW
ESR
10
4
1
⋅
⋅
⋅
f
LC
-------- - R
V
V
OUT
IN
S
⋅
1
Application information
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