AN734
Vishay Siliconix
dling capability of Si9181 is enhanced by the low
junction-to-lead thermal resistance.
LDO OPERATION
Choosing the right series pass element for the regulator is key
to achieving low dropout voltages and high efficiency. LDO
regulators are functionally no different than variable resistance
series elements, where the voltage drop across the pass ele-
ment is equal to the input-to-output voltage differential. The
variable resistor can either be a bipolar transistor, controlled by
the base current, or a MOS transistor, which is controlled by
gate voltage. In bipolar transistors the collector current is con-
trolled by the base current, a process that contributes to a high-
er ground current, diminishing the efficiency of the regulator.
0
V
DS
–V
GS1
–V
GS2
–V
GS3
For CMOS LDOs the series pass element operates in two dif-
ferent regions depending upon the input voltage. If the input
voltage is significantly higher than output voltage, the power
MOSFET operates in the saturation region and acts as a con-
trollable current source. This drain current is a function of the
dID
Saturation Region
Linear Region
transconductance of the power MOSFET, Gm +
, where
I
DS
dVGS
the gate voltage is set by the error amplifier. If the input voltage
decreases to (VOUT + (RON * IOUT)), the voltage regulator can-
not maintain a regulated output. If the input voltage falls below
this voltage, the p-channel power MOSFET enters the linear
region. In this linear region, the gate voltage changes with the
input voltage, and is not a function of the output or control volt-
age. All the circuits in the Si9181 are designed to operate with
inputs as low as 2 V and, on adjustable Si9181 regulators, out-
puts can be programmed as low as 1.5 V.
FIGURE 2. I-V Characteristic of P-Channel MOSFET
POWER DISSIPATION/JUNCTION TEMPERATURE
The Si9181 is rated to deliver up to 600-mA peak current for
2 ms. The maximum load current is specified for continuous
operation and for finite pulse widths. Maximum allowable junc-
tion temperature, junction-to-ambient thermal impedance at a
thermal equilibrium, and the ambient temperature determine
the continuous current rating at a given input-to-output differ-
ential. The input voltage, p-channel power MOSFET transcon-
ductance, and the transient thermal impedance between
junction to lead are major issues for the peak current amplitude
and the pulse width. The maximum continuous power dissipa-
tion allows for a safe junction temperature and is calculated us-
ing the following equation:
I–V CHARACTERISTIC OF SI9181
The Si9181 uses a p-channel power MOSFET series element
that significantly reduces ground current. The Si9181 also fa-
cilitates higher peak current capabilities than solutions using
bipolar series elements, where the maximum current drawn is
limited by the base current. Because the Si9181 uses a p-
channel power MOSFET series element, the gate voltage con-
trols the drain current.
ǒ
Ǔ
(1)
(2)
PD
+
VIN * VOUT IOUT ) VIN IGND
VOUT IOUT
Efficiency +
ǒ
Ǔ
The maximum gate-to-source voltage (VGS) is the same as the
input voltage at any load or input to output voltage differential.
For regulator operation, the gate-to-source voltage should be
high enough to operate the p-channel MOSFET in the satura-
tion region. The available drain current is proportional to the
square of the difference between the applied gate voltage and
transistor threshold voltage. The Si9181 gate threshold is
0.8 V and needs minimum VGS equal to 1.8 V to produce the
600-mA peak current. The 600-mA peak current can be drawn
for at least 2 ms. The package can handle the power dissi-
pated during the peak current pulse period. The power han-
VIN IOUT ) IGND
ǒ
Ǔ
150 * TA
(3)
PDMAX
+
qJA
Where, θJA = 120_C/W (All leads soldered to PC board on 1-oz
copper)
Document Number: 71337
31-Oct-00
www.vishay.com
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