L5987TR STMicroelectronics, L5987TR Datasheet - Page 20

• Manufacturer Part Number: L5987TR
• Description: IC REG SW 3A STEPDOWN 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5987TR

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 3A
• Frequency - Switching: 250kHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• 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 %
• For Use With: 497-8369 - BOARD EVALUATION FOR L5987
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-8494-2

Application information
6.4
20/41
Compensation network
The compensation network has to assure stability and good dynamic performance. The loop
of the L5987 is based on the voltage mode control. The error amplifier is a voltage
operational amplifier with high bandwidth. So selecting the compensation network the E/A
will be considered as ideal, that is, its bandwidth is much larger than the system one.
The transfer functions of PWM modulator and the output LC filter are studied (see
11.). The transfer function of the PWM modulator, from the error amplifier output (COMP
pin) to the OUT pin, results:
Equation 14
where V
generates a sawtooth amplitude directly proportional to the input voltage, that is:
Equation 15
In this way the PWM modulator gain results constant and equals to:
Equation 16
The synchronization of the device with an external clock provided trough SYNCH pin can
modifies the PWM modulator gain (see
and how to keep it constant in spite of the external synchronization).
Figure 10. The error amplifier, the PWM modulation and the LC output filter
The transfer function on the LC filter is given by:
V
V
FB
FB
REF
REF
S
is the sawtooth amplitude. As seen in
E/A
E/A
V
V
S
S
COMP
COMP
Doc ID 14972 Rev 3
G
G
PW0
PW0
G
PW0
PWM
PWM
Chapter 5.1
G
V
S
PW0
=
=
V
-------- -
V
IN
K V
s
=
⋅
Chapter
=
V
-------- -
V
IN
IN
s
--- -
K
1
to understand how this gain changes
=
9
V
V
5.1, the voltage feed forward
CC
CC
OUT
OUT
G
G
LC
LC
L
L
ESR
ESR
C
C
OUT
OUT
Figure
L5987