L6910G データシートの表示（PDF） - STMicroelectronics
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L6910G Datasheet PDF : 26 Pages
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L6910G
Overcurrent protection is performed by the device comparing the drop across the high side MOS, due to the
RDSON, with the voltage across the external resistor (ROCS) connected between the OCSET pin and drain of the
upper MOS. Thus the overcurrent threshold (IP) can be calculated with the following relationship:
IP
=
R-----O----C----S-----⋅---I--O----C----S-
RdsON
Where the typical value of IOCS is 200µA. To calculate the ROCS value it must be considered the maximum
RdsON (also the variation with temperature) and the minimum value of IOCS. To avoid undesirable trigger of
overcurrent protection this relationship must be satisfied:
IP
≥
IOUTMAX
+
∆----I
2
=
IPEAK
Where ∆I is the inductance ripple current and IOUTMAX is the maximum output current.
In case of over current detectionthe soft start capacitor is discharged with constant current (10µA typ.) and when
the SS pin reaches 0.5V the soft start phase is restarted. During the soft start the over-current protection is al-
ways active and if such kind of event occurs, the device turns off both mosfets, and the SS capacitor is dis-
charged again (after reaching the upper threshold of about 4V). The system is now working in HICCUP mode,
as shown in figure 8. After removing the cause of the over-current, the device restart working normally without
power supplies turn off and on.
Figure 8. Hiccup Mode
Figure 9. Inductor Ripple Current vs. Vout
CH1 = SS; CH4 = Inductor current
9
8
7
6
5
4
3
2
1
0
0 .5
L=1.5µH, Vin=12V
L=2µH,
Vin=12V
L=3µH,
Vin=12V
L=1.5µH,
Vin=5V
L=2µH,
Vin=5V
L=3µH, Vin=5V
1 .5
2 .5
3 .5
Output Voltage [V]
4.6 Inductor Design
The inductance value is defined by a compromise between the transient response time, the efficiency, the cost
and the size. The inductor has to be calculated to sustain the output and the input voltage variation to maintain
the ripple current ∆IL between 20% and 30% of the maximum output current. The inductance value can be cal-
culated with this relationship:
L
=
-V----I-N-----–-----V----O----U----T--
fsw ⋅ ∆IL
⋅
V-----O----U----T-
VIN
Where fSW is the switching frequency, VIN is the input voltage and VOUT is the output voltage. Figure 9 shows
the ripple current vs. the output voltage for different values of the inductor, with VIN = 5V and VIN = 12V.
Increasing the value of the inductance reduces the ripple current but, at the same time, reduces the converter
response time to a load transient. If the compensation network is well designed, the device is able to open or
close the duty cycle up to 100% or down to 0%. The response time is now the time required by the inductor to
change its current from initial to final value. Since the inductor has not finished its charging time, the output cur-
rent is supplied by the output capacitors. Minimizing the response time can minimize the output capacitance
required.
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