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LT3502AEMS#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3502/LT3502A
10
3502fd
applications inForMation
Figure 1. Continuous Mode Operation Near
Minimum On-Time of 60ns
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis
-
tors according to:
R1= R2
V
OUT
0.8V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for the LT3502/LT3502A applica
-
tions depends on the output voltage and on the absolute
maximum ratings of the V
IN
and BOOST pins.
The minimum input voltage is determined by either the
LT3502/LT3502A’s minimum operating voltage of 3V, or
by its maximum duty cycle. The duty cycle is the fraction
of time that the internal switch is on and is determined
by the input and output voltages:
DC =
V
OUT
+
V
D
V
IN
– V
SW
+ V
D
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.45V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
=
V
OUT
+
V
D
DC
MAX
– V
D
+ V
SW
with DC
MAX
= 0.80 for the LT3502A and 0.90 for the
LT3502.
The maximum input voltage is determined by the
absolute maximum ratings of the V
IN
and BOOST pins. For
fixed frequency operation, the maximum input voltage is
determined by the minimum duty cycle DC
MIN
:
V
IN(MAX)
=
V
OUT
+
V
D
DC
MIN
– V
D
+ V
SW
DC
MIN
= 0.15 for the LT3502A and 0.08 for the LT3502.
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 33V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 250mA
3502 F01
I
L
500mA/DIV
Note that this is a restriction on the operating input volt-
age for fixed frequency operation; the circuit will tolerate
transient inputs up to the absolute maximum ratings of
the V
IN
and BOOST pins. The input voltage should be
limited to the V
IN
operating range (40V) during overload
conditions.
Minimum On-Time
The LT3502/LT3502A will still regulate the output at input
voltages that exceed V
IN(MAX)
(up to 40V), however, the
output voltage ripple increases as the input voltage is
increased.
As the input voltage is increased, the part is required to
switch for shorter periods of time. Delays associated with
turning off the power switch dictate the minimum on-time
of the part. The minimum on-time for the LT3502/LT3502A
is 60ns (Figure 1).
When the required on-time decreases below the mini
-
mum on-time of 60ns, instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the switch pulse width remains fixed at
60ns. The inductor current ramps up to a value exceed
-
ing the load current and the output ripple increases. The
pa
rt then remains off until the output voltage dips below
the programmed value before it begins switching again
(Figure 2).
Provided that the load can tolerate the increased output
voltage ripple and that the components have been properly
selected, operation above V
IN(MAX)
is safe and will not
damage the part.
LT3502/LT3502A
11
3502fd
Figure 2. Pulse-Skipping Occurs when
Required On-Time is Below 60ns
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 40V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 250mA
3502 F02
I
L
500mA/DIV
applications inForMation
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 1.6(V
OUT
+ V
D
) for the LT3502A
L = 4.6(V
OUT
+ V
D
) for the LT3502
where V
D
is the voltage drop of the catch diode (~0.4V) and
L is in µH. With this value there will be no subharmonic
oscillation for applications with 50% or greater duty cycle.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be about 30% higher. For robust operation during fault
conditions, the saturation current should be above 1.2A.
To keep efficiency high, the series resistance (DCR) should
be less than 0.1Ω. Table 1 lists several vendors and types
that are suitable.
There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
inductance may result in discontinuous mode opera
-
Figure 3. Pulse-Skipping with Large Load Current Will be
Limited by the DA Valley Current Limit. Notice the Flat Inductor
Valley Current and Reduced Switching Frequency
As the input voltage increases, the inductor current ramps
up quicker, the number of skipped pulses increases and
the output voltage ripple increases. For operation above
V
IN(MAX)
the only component requirement is that the
components be adequately rated for operation at the
intended voltage levels.
Inductor current may reach current limit when operating
in
pulse-skipping
mode with small valued inductors. In
this case, the LT3502/LT3502A will periodically reduce its
frequency to keep the inductor valley current to 650mA
(Figure 3). Peak inductor current is therefore peak current
plus minimum switch delay:
900mA +
V
IN
– V
OUT
L
• 60ns
The part is robust enough to survive prolonged operation
under these conditions as long as the peak inductor cur-
rent does not exceed 1.2A. Inductor current saturation
and junction temperature may further limit per
formance
during this operating regime.
V
SW
20V/DIV
V
OUT
100mV/DIV
1µs/DIV
V
IN
= 40V, V
OUT
= 3.3V
L = 6.8µH, C
OUT
= 10µF, I
OUT
= 500mA
3502 F03
I
L
500mA/DIV
Table 1
VENDOR URL PART SERIES INDUCTANCE RATE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH8D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5 ×
4.5
5.5 × 5.5
8.3 × 8.3
Toko www.toko.com A916CY
D585LC
2 to 12
1.1 to 39
6.3 ×
6.2
8.1 × 8
Würth Elektronik www.we-online.com WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8 ×
4.8
5.2 × 5.8
7.3 × 7.3
LT3502/LT3502A
12
3502fd
applications inForMation
tion, which is okay, but further reduces maximum load
current. For details of the maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Catch Diode
A low capacitance 500mA Schottky diode is recommended
for the catch diode, D1. The diode must have a reverse
voltage rating equal to or greater than the maximum input
voltage. The Diodes Inc. SBR1U40LP, ON Semi MBRM140,
and Diodes Inc. DFLS140 are good choices for the catch
diode.
Input Capacitor
Bypass the input of the LT3502/LT3502A circuit with a 1µF
or higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and applied
voltage and should not be used. A 1µF ceramic is adequate
to bypass the LT3502/LT3502A and will easily handle the
ripple current. However, if the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca
-
pacitor is required to reduce the resulting voltage ripple at
the LT3502/LT3502A and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1µF capacitor is capable of this task, but only if it is placed
close to the LT3502/LT3502A and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input volt
-
age rating of the LT3502/LT3502A. A ceramic input capaci-
tor combined with trace or cable inductance forms a high
quality (underdamped) tank cir
cuit. If the L
T3502/LT3502A
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3502/LT3502A’s voltage rating. This situation is easily
avoided; see the Hot Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the LT3502/LT3502A to produce the DC output. In this role
it determines the output ripple so low impedance at the
switching frequency is important. The second function is to
store energy in order to satisfy transient loads and stabilize
the LT3502/LT3502A’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. A good value is:
C
OUT
=
33
V
OUT
for the LT3502A
C
OUT
=
66
V
OUT
for the LT3502
where C
OUT
is in µF. Use an X5R or X7R type and keep
in mind that a ceramic capacitor biased with V
OUT
will
have less than its nominal capacitance. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor, R1, may be required to get the full benefit (see
the Compensation section).
For small size, the output capacitor can be chosen
according to:
C
OUT
=
25
V
OUT
where C
OUT
is in µF. However, using an output capacitor
this small results in an increased loop crossover frequency
and increased sensitivity to noise.
High performance electrolytic capacitors can be used for
the output capacitor. Low ESR is important, so choose
one that is intended for use in switching regulators. The
ESR should be specified by the supplier and should be
0.1Ω or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
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LT3502AEMS#TRPBF

Mfr. #:
Buy LT3502AEMS#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 750kHz/2.2MHz,500mA Step-Down Regulator
Lifecycle:
New from this manufacturer.
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