L5987 STMicroelectronics, L5987 Datasheet - Page 26

• Manufacturer Part Number: L5987
• Description: IC REG STEPDOWN 3A 8VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

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

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