LT1507
APPLICATIONS INFORMATION
committing to a final design. The fixed 3.3V LT1507-3.3
has internal divider resistors and the FB pin is renamed
SENSE, connected directly to the output.
The suggested value for the output divider resistor from FB
to ground (R2) is 5k or less and the formula for R1 is
shown below. The output voltage error caused by ignoring
the input bias current on the FB pin is less than 0.25% with
R2 = 5k. Please read below if R2 is increased above the
suggested value.
R1
=
R2(V
OUT
– 2.42)
2.42
More Than Just Voltage Feedback
The feedback pin is used for more than just output voltage
sensing. It also reduces switching frequency and current
limit when output voltage is very low (see graph in Typical
Performance Characteristics). This is done to control
power dissipation in both the IC and in the external diode
and inductor during short-circuit conditions. A shorted
output requires the switching regulator to operate at very
low duty cycles and the average current through the diode
and inductor is equal to the short-circuit current limit of the
switch (typically 2A of the LT1507, folding back to less
than 1A). Minimum switch ON time limitations would
prevent the switcher from attaining a sufficiently low duty
cycle if switching frequency were maintained at 500kHz,
so frequency is reduced by about 5:1 when the feedback
V
IN
C3*
33µF
20V
TANTALUM
V
IN
+
DEFAULT
(OPEN)
= ON
SHDN
GND
* AVX TPSD337M020R0200 OR SPRAGUE 593 EQUIVALENT.
RIPPLE CURRENT RATING
≥
0.6A
** AVX TPSD108M010R0100 OR SPRAGUE 593 EQUIVALENT
*** COILTRONICS CTX5-1. SUBSTITUTION UNITS SHOULD BE RATED
AT
≥
1.25A, USING LOW LOSS CORE MATERIAL. LOAD CURRENTS
ABOVE 0.85A MAY NEED A 10µH OR 20µH INDUCTOR
Figure 2. Typical Schematic for LT1507 Adjustable Application
8
U
W
U
U
pin voltage drops below 1V (see Frequency Foldback
graph). This does not affect operation with normal load
conditions; one simply sees a gear shift in switching
frequency during start-up as the output voltage rises.
In addition to lower switching frequency, the LT1507 also
operates at lower switch current limit when the feedback
pin voltage drops below 1.5V. This
foldback current limit
greatly reduces power dissipation in the IC, diode and
inductor during short-circuit conditions. Again, it is nearly
transparent to the user under normal load conditions. The
only loads which may be affected are current source loads
which maintain full-load current with output voltage less
than 50% of final value. In these rare situations, the
feedback pin can be clamped above 1.5V with an external
diode to defeat foldback current limit.
Caution:
clamping
the feedback pin means that frequency shifting will also be
defeated, so a combination of high input voltage and dead
shorted output may cause the LT1507 to lose control of
current limit.
The internal circuitry which forces reduced switching
frequency also causes current to flow out of the feedback
pin when output voltage is low. If the FB pin falls below 1V,
current begins to flow out of the pin and reduces frequency
at the rate of approximately 5kHz/µA. To ensure adequate
frequency foldback (under worst-case short-circuit con-
ditions) the external divider Thevinin resistance must be
low enough to pull 150µA out of the FB pin with 0.6V on the
D2
1N914
C2
0.1µF
BOOST
V
SW
LT1507
FB
V
C
C
C
3.3nF
D1
1N5818
R2
4.99k
L1***
5µH
R1
5.36k
OUTPUT
5V
+
C1**
100µF
10V
TANTALUM
LT1507 • F01
LINER [ LINEAR TECHNOLOGY ]
