L5981 STMicroelectronics, L5981 Datasheet - Page 17

• 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
5.4
Compensation network
The compensation network has to assure stability and good dynamic performance. The loop
of the L5981 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
The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to
the OUT pin, results:
Equation 10
where V
generates a sawtooth amplitude directly proportional to the input voltage, that is:
Equation 11
In this way the PWM modulator gain results constant and equals to:
Equation 12
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 9.
The transfer function on the LC filter is given by:
V
V
FB
FB
REF
REF
S
is the sawtooth amplitude. As seen in
The error amplifier, the PWM modulator and the LC output filter
E/A
E/A
V
V
S
S
COMP
COMP
Doc ID 13004 Rev 6
G
G
PW0
PW0
G
PW0
PWM
PWM
Chapter 4.1
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
4.1, the voltage feed forward
CC
CC
OUT
OUT
Application information
G
G
LC
LC
L
L
ESR
ESR
C
C
OUT
OUT
Figure
17/37
9).