L5986 Datasheet, PDF (17/39 Pages) STMicroelectronics – 2.5 A step-down switching regulator

L5986 - L5986A
Application information
6.4
Table 8. Output capacitors
Manufacturer
Series
MURATA
GRM32
GRM31
PANASONIC
SANYO
ECJ
EEFCD
TPA/B/C
TDK
C3225
Cap value (μF)
22 to 100
10 to 47
10 to 22
10 to 68
100 to 470
22 to 100
Rated voltage (V)
6.3 to 25
6.3 to 25
6.3
6.3
4 to 16
6.3
ESR (mΩ)
<5
<5
<5
15 to 55
40 to 80
<5
Compensation network
The compensation network has to assure stability and good dynamic performance. The loop
of the L5986 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 Figure
9.). The transfer function of the PWM modulator, from the Error Amplifier output (COMP pin)
to the OUT pin, results:
Equation 10
G_PW0
=
V_-----I-N--
V_s
where V_S is the sawtooth amplitude. As seen in Chapter 5.1, the voltage feed forward
generates a sawtooth amplitude directly proportional to the input voltage, that is:
Equation 11
V_S = K ⋅ V_IN
In this way the PWM modulator gain results constant and equals to:
Equation 12
G_PW0
=
V_-----I-N--
V_s
=
-¹--
K
=
9
The synchronization of the device with an external clock provided trough SYNCH pin can
modifies the PWM modulator gain (see Chapter 5.1 to understand how this gain changes
and how to keep it constant in spite of the external synchronization).
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