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LT1506CR-3.3#TRPBF

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7
LT1506
BLOCK DIAGRAM
W
and output capacitor, then an abrupt 180° shift will occur.
The current fed system will have 90° phase shift at a much
lower frequency, but will not have the additional 90° shift
until well beyond the LC resonant frequency. This makes
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode. Two comparators are connected to the
shutdown pin. One has a 2.38V threshold for undervoltage
lockout and the second has a 0.4V threshold for complete
shutdown.
The LT1506 is a constant frequency, current mode buck
converter. This means that there is an internal clock and
two feedback loops that control the duty cycle of the power
switch. In addition to the normal error amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscilla-
tor pulse which sets the R
S
flip-flop to turn the switch on.
When switch current reaches a level set by the inverting
input of the comparator, the flip-flop is reset and the
switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
Figure 1. Block Diagram
–
+
–
+
Σ
INPUT
2.9V BIAS
REGULATOR
500kHz
OSCILLATOR
FREQUENCY
SHIFT CIRCUIT
V
SW
FB
V
C
GND
1506 BD
SLOPE COMP
0.01Ω
INTERNAL
V
CC
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 20
SYNC
SHDN
SHUTDOWN
COMPARATOR
LOCKOUT
COMPARATOR
CURRENT
COMPARATOR
ERROR
AMPLIFIER
g
m
= 2000µMho
FOLDBACK
CURRENT
LIMIT
CLAMP
BOOST
R
S
FLIP-FLOP
DRIVER
CIRCUITRY
S
R
0.9V
Q2
Q1
POWER
SWITCH
PARASITIC DIODES
DO NOT FORWARD BIAS
2.42V
–
+
0.4V
3.5µA
2.38V
8
LT1506
APPLICATIONS INFORMATION
WUU
U
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1506 is used to set output
voltage and provide several overload protection features.
The first part of this section deals with selecting resistors
to set output voltage and the remaining part talks about
foldback frequency and current limiting created by the FB
pin. Please read both parts before committing to a final
design. The fixed 3.3V LT1506-3.3 has internal divider
resistors and the FB pin is renamed SENSE, connected
directly to the output.
The suggested value for the output divider resistor (see
Figure 2) from FB to ground (R2) is 5k or less, and a
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 the following
if divider resistors are increased above the suggested
values.
R
RV
OUT
1
2242
242
=
−
()
.
.
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 the Frequency
Foldback 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 regu-
lator 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 6A for the
LT1506, folding back to less than 3A). 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 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 LT1506 also
operates at lower switch current limit when the feedback
pin voltage drops below 1.7V. Q2 in Figure 2 performs this
function by clamping the V
C
pin to a voltage less than its
normal 2.1V upper clamp level. This
foldback current limit
greatly reduces power dissipation in the IC, diode and
inductor during short-circuit conditions. External synchro-
nization is also disabled to prevent interference with
foldback operation. Again, it is nearly transparent to the
user under normal load conditions. The only loads that 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 cur-
rent 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 LT1506 to lose control of current limit.
Figure 2. Frequency and Current Limit Foldback
–
+
2.4V
V
SW
V
C
GND
TO SYNC CIRCUIT
1506 F02
TO FREQUENCY
SHIFTING
R3
1k
R4
1k
R1
R2
5k
OUTPUT
5V
R5
5k
ERROR
AMPLIFIER
FB
1.6V
Q1
LT1506
Q2
+
9
LT1506
APPLICATIONS INFORMATION
WUU
U
The internal circuitry which forces reduced switching
frequency also causes current to flow out of the feedback
pin when output voltage is low. The equivalent circuitry is
shown in Figure 2. Q1 is completely off during normal
operation. If the FB pin falls below 1V, Q1 begins to
conduct current and reduces frequency at the rate of
approximately 5kHz/µA. To ensure adequate frequency
foldback (under worst-case short-circuit conditions), the
external divider Thevinin resistance must be low enough
to pull 150µA out of the FB pin with 0.6V on the pin (R
DIV
≤ 4k).
The net result is that reductions in frequency and
current limit are affected by output voltage divider imped-
ance. Although divider impedance is not critical, caution
should be used if resistors are increased beyond the
suggested values and short-circuit conditions will occur
with high input voltage
. High frequency pickup will
increase and the protection accorded by frequency and
current foldback will decrease.
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
P
) of the LT1506.
This current rating is 4.5A up to 50% duty cycle (DC),
decreasing to 3.7A at 80% duty cycle. This is shown
graphically in Typical Performance Characteristics and as
shown in the formula below:
I
P
= 4.5A for DC ≤ 50%
I
P
= 3.21 + 5.95(DC) – 6.75(DC)
2
for 50% < DC < 90%
DC = Duty cycle = V
OUT
/V
IN
Example: with V
OUT
= 5V, V
IN
= 8V; DC = 5/8 = 0.625, and;
I
SW(MAX)
= 3.21 + 5.95(0.625) – 6.75(0.625)
2
= 4.3A
Current rating decreases with duty cycle because the
LT1506 has internal slope compensation to prevent cur-
rent mode subharmonic switching. For more details, read
Application Note 19. The LT1506 is a little unusual in this
regard because it has nonlinear slope compensation which
gives better compensation with less reduction in current
limit.
Maximum load current would be equal to maximum
switch current
for an infinitely large inductor
, but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current. The following
formula assumes continuous mode operation, implying
that the term on the right is less than one-half of I
P
.
I
OUT(MAX)
=
Continuous Mode
For the conditions above and L = 3.3µH,
I
A
OUT MAX
(
)
−
=−
()
−
()
()
=− =
43
58 5
2 3 3 10 500 10 8
43 057 373
63
.
.• •
.. .
At V
IN
= 15V, duty cycle is 33%, so I
P
is just equal to a fixed
4.5A, and I
OUT(MAX)
is equal to:
45
515 5
2 3 3 10 500 10 15
45 101 349
63
.
.• •
.. .
−
()
−
()
()
=− =
−
A
Note that there is less load current available at the higher
input voltage because inductor ripple current increases.
This is not always the case. Certain combinations of
inductor value and input voltage range may yield lower
available load current at the lowest input voltage due to
reduced peak switch current at high duty cycles. If load
current is close to the maximum available, please check
maximum available current at both input voltage ex-
tremes. To calculate actual peak switch current with a
given set of conditions, use:
II
VVV
LfV
SW PEAK OUT
OUT IN OUT
IN
(
)
=+
−
()
()()( )
2
I
P
−
()
−
()
()()( )
VVV
LfV
OUT IN OUT
IN
2
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LT1506CR-3.3#TRPBF

Mfr. #:
Buy LT1506CR-3.3#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 4.5A, 500kHz Buck Sw Reg
Lifecycle:
New from this manufacturer.
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