LT1033_02 Datasheet, PDF (5/12 Pages) Linear Technology – 3A Negative Adjustable Regulator

LT1033
APPLICATIONS INFORMATION
Output Voltage
The output voltage is determined by two external resis-
tors, R1 and R2 (see Figure 1). The exact formula for the
output voltage is:
V_OUT = V_REF^{}1+ ^{RR21} + I_ADJ(R2₎
Where: V_REF = Reference Voltage, I_ADJ = Adjustment Pin
Current. In most applications, the second term is small
enough to be ignored, typically about 0.5% of V_OUT. In
more critical applications, the exact formula should be
used, with I_ADJ equal to 65µA. Solving for R2 yields:
R2
=
V_OUT
V_REF
R1
– V_REF
– I_ADJ
Smaller values of R1 and R2 will reduce the influence of
I_ADJ on the output voltage, but the no-load current drain on
the regulator will be increased. Typical values for R1 are
between 100Ω and 300Ω, giving 12.5mA and 4.2mA
no-load current respectively. There is an additional con-
sideration in selecting R1, the minimum load current
specification of the regulator. The operating current of the
LT1033 flows from input to output. If this current is not
absorbed by the load, the output of the regulator will rise
above the regulated value. The current drawn by R1 and R2
is normally high enough to absorb the current, but care
must be taken in no-load situations where R1 and R2 have
high values. The maximum value for the operating current,
C1
10µF _IADJ
R2
C3
C2
5µF
2µF
V_REF R1
ADJ
–V_IN
V_IN
V_OUT
LT1033
–V_OUT
LT1033 • F01
EXAMPLE:
1. A PRECISION 10V REGULATOR TO SUPPLY UP TO 3A LOAD CURRENT.
A. SELECT R1 = 100Ω TO MINIMIZE EFFECT OF I_ADJ
B. CALCULATE R2 = ^{VOUT – VREF} = ^{10V – 1.25V} = 704Ω
V_REF
R1
–
I_ADJ
1.25V
100Ω
– 65µA
Figure 1
which must be absorbed, is 5mA for the LT1033. If input-
output voltage differential is less than 10V, the operating
current that must be absorbed drops to 3mA.
Capacitors and Protection Diodes
An output capacitor, C3, is required to provide proper
frequency compensation of the regulator feedback loop.
A 2µF or larger solid tantalum capacitor is generally
sufficient for this purpose if the 1MHz impedance of the
capacitor is 1Ω or less. High Q capacitors, such as Mylar,
are not recommended because their extremely low ESR
(effective series resistance) can drastically reduce phase
margin. When these types of capacitors must be used
because of other considerations, add a 0.5Ω carbon
resistor in series with 1µF. Aluminum electrolytic capaci-
tors may be used, but the minimum value should be 25µF
to ensure a low impedance at 1MHz. The output capacitor
should be located within a few inches of the regulator to
keep lead impedance to a minimum. The following caution
should be noted: if the output voltage is greater than 6V
and an output capacitor greater than 20µF has been used,
it is possible to damage the regulator if the input voltage
becomes shorted, due to the output capacitor discharging
into the regulator. This can be prevented by using diode D1
(see Figure 2) between the input and the output.
The input capacitor, C2, is only required if the regulator is
more than 4 inches from the raw supply filter capacitor.
Bypassing the Adjustment Pin
The adjustment pin of the LT1033 may be bypassed with
a capacitor to ground, C1, to reduce output ripple, noise,
and impedance. These parameters scale directly with
output voltage if the adjustment pin is not bypassed. A
bypass capacitor reduces ripple, noise and impedance to
that of a 1.25V regulator. In a 15V regulator for example,
these parameters are improved by 15V/1.25V = 12 to 1.
This improvement holds only for those frequencies where
the impedance of the bypass capacitor is less than R1. Ten
microfarads is generally sufficient for 60Hz power line
applications where the ripple frequency is 120Hz, since
X_C = 130Ω. The capacitor should have a voltage rating at
least as high as the output voltage of the regulator. Values
1033fc
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