L5985 STMicroelectronics, L5985 Datasheet - Page 20

• Manufacturer Part Number: L5985
• Description: IC REG SW STEP DOWN 2A 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5985

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 2A
• 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: 2 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: 497-6387 - BOARD EVAL FOR L5985
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application information
20/37
Figure 11. 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 18
where K is the feed forward constant and 1/K is equals to 9.
3.
Equation 19
4.
Equation 20
5.
Equation 21
Choose a value for R
Choose a gain (R
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
5
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):
4
/R
R
1
1
) in order to have the required bandwidth (BW), that means:
, usually between 1 k and 5 k.
3
=
-------------------------- -
4 BW
----------------- 1
Doc ID 13006 Rev 5
⋅
f
C
LC
5
R
1
=
–
R
------------------------------------------------------------- -
2π R
C
4
4
,
⋅
=
=
BW K
----------------- - R
4
-------------------------- -
π R
⋅
f
⋅
LC
C
⋅
C
C
4
1
4
4
⋅
3
⋅
4 BW
⋅
=
f
LC
⋅
---------------------------------------- -
2π R
1
⋅
–
3
1
1
⋅
4 BW
⋅
L5985
LC
):