LT1507
U
W U U
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.
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.
ThesuggestedvaluefortheoutputdividerresistorfromFB
to ground (R2) is 5k or less and the formula for R1 is
shownbelow. Theoutputvoltageerrorcausedbyignoring
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.
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
thefeedbackpinmeansthatfrequencyshiftingwillalsobe
defeated, so a combination of high input voltage and dead
shorted output may cause the LT1507 to lose control of
current limit.
R2(V
– 2.42)
2.42
OUT
R1 =
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
andinductorisequaltotheshort-circuitcurrentlimitofthe
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
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,
currentbeginstoflowoutofthepinandreducesfrequency
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
lowenoughtopull150µAoutoftheFBpinwith0.6Vonthe
D2
1N914
C2
0.1µF
BOOST
OUTPUT
V
V
V
IN
IN
SW
FB
5V
L1***
R1
LT1507
C3*
33µF
20V
5µH
+
5.36k
DEFAULT
(OPEN)
= ON
SHDN
GND
TANTALUM
V
C
C1**
100µF
10V
+
D1
1N5818
R2
4.99k
C
3.3nF
C
TANTALUM
AVX TPSD337M020R0200 OR SPRAGUE 593 EQUIVALENT.
RIPPLE CURRENT RATING ≥ 0.6A
*
LT1507 • F01
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
Linear [ Linear ]
