L5987 STMicroelectronics, L5987 Datasheet - Page 26
L5987
Manufacturer Part Number
L5987
Description
IC REG STEPDOWN 3A 8VFQFPN
Manufacturer
STMicroelectronics
Type
Step-Down (Buck)r
Datasheet
1.L5987TR.pdf
(41 pages)
Specifications of L5987
Internal Switch(s)
Yes
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
0.6 V ~ 18 V
Current - Output
3A
Frequency - Switching
250kHz
Voltage - Input
2.9 V ~ 18 V
Operating Temperature
-40°C ~ 150°C
Mounting Type
Surface Mount
Package / Case
8-VFQFN, Exposed Pad
Output Voltage
0.6 V
Output Current
3 A
Output Power
1.5 W
Input Voltage
2.9 V to 18 V
Switching Frequency
250 KHz
Mounting Style
SMD/SMT
Duty Cycle (max)
100 %
Low Dropout Operation
100 % duty cycle
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
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Application information
26/41
Figure 15. 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 26
Where f
Equation 27
and Vs is the saw-tooth amplitude. The voltage feed forward keeps the ratio Vs/Vin constant.
3.
Equation 28
4.
Choose a value for R
and 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 kΩ and 5 kΩ, in order to have values of C4
Doc ID 14972 Rev 3
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
L5987