L4915 데이터 시트보기 (PDF) - STMicroelectronics
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L4915 Datasheet PDF : 7 Pages
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L4915
ELECTRICAL CHARACTERISTICS (continued)
(Tamb = 25oC; Vi = 13.5 V, VO = 8.5V, circuit of Fig. 1, unless otherwise specified)
Symbol
∆VO
∆T
SVR
Parameter
Output Voltage Drift
Supply Voltage Rejection
ISC Short Circuit Current
Ton Switch On Time
Tj
Thermal Shutdown Junction
Temperature
Test Conditions
Io = 10mA
Viac = 1VRMS, f = 100Hz, Io = 150mA
Regulator
Filter Mode
Io = 150mA
Regulator
Filter Mode
Min. Typ. Max. Unit
1.2
mV/°C
dB
71
35(*)
250 300
mA
ms
300
500(*)
145
°C
(*) Depending of the CFT capacitor
PRINCIPLE OF OPERATION
During normal operation (input voltage upper than
VI MIN = VOUT NOM + ∆VI/O). The device works as a
normal voltage regulator built around the OP1 of
the block diagram.
The series pass element uses a PNP-NPN connec-
tion to reduce the dropout. The reference voltage
of the OP1 is derived from a REF through the OP2
and Q3, acting as an active zener diode of value
VREF.
In this condition the device works in the range (1)
of the characteristic of the non linear drop control
unit (see Figure 1).
Figure 1 : Nonliner Transfer Characteristic of
the Drop Control Unit
The output voltage is fixed to its nominal value :
VOUT
NOM
=
VREF
1
+
R1
R2
=
VCFT
1
+
R1
R2
The ripple rejection is quite high (70dB) and inde-
pendent to CFT value.
On the usual voltage regulators, when the input
vol-tage goes below the nominal value, the regula-
tion transistors (series element) saturate bringing
the system out of regulation and making it very
sensible to every variation of the input voltage. On
the contrary, a control loop on the L4915 consents
to avoid the saturation of the series element by
regulating the value of the reference voltage
(pin 2). In fact, whenever the input voltage de-
creases below (VI MIN the supervisor loop, utilizing
a non linear OTA, forces the reference voltage at
pin 2 to decrease by discharging CFT. So, during
the static mode, when the input voltage goes below
VMIN the drop out is kept fixed to about 1.6 V. In this
condition the device works as a low pass filter in
the range (2) of the OTA characteristic. The ripple
rejection is externally adjustable acting on CFT as
follows :
SVR
(jΩ)
=
Vi (jΩ)
VOUT(jΩ)
=
1
+
10−6
gm
jw CFT
1
+
R1
R2
Where:
gm = 2 ⋅ 10-5 Ω-1
= OTA’S typical transconductance value on
linear region
R1 = fixed ratio
R2
CFT = value of capacitor in µF
The reaction time of the supervisor loop is given by
the transconductanceof the OTAand by CFT. When
the value of the ripple voltage is so high and its
negative peak is fast enough to determine an istan-
taneous decrease of the dropout till 1.2V, the OTA
works in a higher transconductance condition
[range (3) of the characteristic] and discharges the
capacitor rapidously.
If the ripple frequency is high enough the capacitor
won’t charge itself completely, and the output volt-
age reaches a small value allowing a better ripple
rejection ; the device’s again working as a filter (fast
transient range).
With CFT = 10µF; f = 100Hz; Vo = 8.5V a SVR of 35
is obtained.
3/7
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