LT1683IG Ver la hoja de datos (PDF) - Linear Technology
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LT1683IG Datasheet PDF : 26 Pages
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LT1683
APPLICATIONS INFORMATION
The FB bias current represents a small error and can
usually be ignored for values of R1||R2 up to 10k.
One word of caution, sometimes a feedback zero is added
to the control loop by placing a capacitor across R1. If
the feedback capacitively pulls the FB pin above the in-
ternal regulator voltage (2.4V), output regulation may be
disrupted. A series resistance with the feedback pin can
eliminate this potential problem. There is an internal clamp
on FB that clamps at 0.7V above the regulation voltage
that should also help prevent this problem.
FB PIN
R1
VOUT
R2
1683 F03
Figure 3
Negative Output Voltage Setting
Negative output voltage can be sensed using the NFB pin.
In this case regulation will occur when the NFB pin is at
–2.5V. The nominal input bias current for the NFB is
–25µA (INFB), which needs to be accounted for in setting
up the divider.
Referring to Figure 4, R1 is chosen such that:
R1=
R2
VOUT − 2.5
2.5 +R2 • 25µA
A suggested value for R2 is 2.5k. The NFB pin is normally
left open if the FB pin is being used.
NFB PIN
INFB
R1
R2
–VOUT
1683 F04
Figure 4
Dual Polarity Output Voltage Sensing
Certain applications may benefit from sensing both posi-
tive and negative output voltages. When doing this each
output voltage resistor divider is individually set as previ-
ously described. When both FB and NFB pins are used,
the LT1683 will act to prevent either output from going
beyond its set output voltage. The highest output (lightest
load) will dominate control of the regulator. This technique
would prevent either output from going unregulated high
at no load. However, this technique will also compromise
output load regulation.
Shutdown
If SHDN is pulled low, the regulator will turn off. As the
SHDN pin voltage is increased from ground the internal
bandgap regulator will be powered on. This will set a 1.39V
threshold for turn-on of the internal regulator that runs
most of the control circuitry of the regulator. Note after the
control circuitry powers on, gate driver activity will depend
on the voltage of VIN with respect to the voltage on GCL.
As the SHDN pin enables the internal regulator a 24µA
current will be sourced from the pin that can provide
hysteresis for undervoltage lockout. This hysteresis can
be used to prevent part shutdown due to input voltage
sag from an initial high current draw.
In addition to the current hysteresis, there is also approxi-
mately 100mV of voltage hysteresis on the SHDN pin.
When the SHDN pin is greater than 2.2V, the hysteretic
current from the part will be reduced to essentially zero.
If a resistor divider is used to set the turn-on threshold then
the resistors are determined by the following equations:
VON
=
RA +RB
RB
•
VSHDN
RA
VIN
RB
SHDN
VHYST
= RA
•
∆ VSHDN
RA RB
+ISHDN
Reworking these equations yields:
RA
=
(VHYST
• VSHDN
(ISHDN
− VON • ∆
• VSHDN )
VSHDN
)
RB
=
(VHYST • VSHDN −
ISHDN •(VON
VON • ∆ VSHDN
− VSHDN )
)
1683fd
15
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