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LT3500IMSE#TRPBF

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LT3500
16
3500fc
the value of output capacitance required. Also, the current
mode control loop doesn’t require the presence of output
capacitor series resistance (ESR). For these reasons, you
are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. Estimate output ripple
with the following equations:
V
RIPPLE
=
ΔI
L
8 •Frequency •C
OUT1
For ceramic capacitors and,
V
RIPPLE
= ∆I
L
• ESR
For electrolytic (tantalum and aluminum)
where ∆I
L
is the peak-to-peak ripple current in the
inductor.
The RMS content of this ripple is very low, and the RMS
current rating of the output capacitor is usually not of
concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this require-
ment becomes:
C
OUT1
> 10 •L
I
LIM
V
OUT1
2
Finally, there must be enough capacitance for good transient
performance. The last equation gives a good starting point.
Alternatively, you can start with one of the designs in this
data sheet and experiment to get the desired performance.
This topic is covered more thoroughly in the section on
loop compensation.
The high performance (low ESR), small size and robust-
ness of ceramic capacitors make them the preferred type
for LT3500 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the high
value capacitors use poor dielectrics with high tempera-
ture and voltage coeffi cients. In particular, Y5V and Z5U
types lose a large fraction of their capacitance with ap-
plied voltage and temperature extremes. Because the loop
stability and transient response depend on the value of
C
OUT1
, you may not be able to tolerate this loss. Use X7R
and X5R types. You can also use electrolytic capacitors.
The ESRs of most aluminum electrolytics are too large to
deliver low output ripple. Tantalum and newer, lower ESR
organic electrolytic capacitors intended for power supply
use, are suitable and the manufacturers will specify the
ESR. The choice of capacitor value will be based on the
ESR required for low ripple. Because the volume of the
capacitor determines its ESR, both the size and the value
will be larger than a ceramic capacitor that would give you
similar ripple performance. One benefi t is that the larger
capacitance may give better transient response for large
changes in load current.
Catch Diode
The diode D1 conducts current only during switch off
time. Use a Schottky diode to limit forward voltage drop to
increase effi ciency. The Schottky diode must have a peak
reverse voltage that is equal to regulator input voltage and
sized for average forward current in normal operation.
Average forward current can be calculated from:
I
D(AVG)
=
I
OUT1
V
IN
•V
IN
− V
OUT1
()
APPLICATIONS INFORMATION
LT3500
17
3500fc
Figure 5. BST Pin Considerations
V
BST
– V
SW
= V
X
V
BST(MAX)
= V
X
V
X
> V
IN
+ 3V
(5d)(5c)
(5b)
V
BST
– V
SW
= V
IN
V
BST(MAX)
= 2 •V
IN
V
IN
V
IN
D2
D1
D1
D1
D1
(5a)
LT3500
LDRV
BST
SW
V
BST
– V
SW
= V
OUT1
V
BST(MAX)
= V
IN
+ V
OUT1
V
BST
– V
SW
= V
OUT2
V
BST(MAX)
= V
IN
+ V
OUT2
V
OUT2
≥ 2.5V
V
OUT1
V
OUT2
C3
V
IN
V
IN
D2
LT3500
LDRV
BST
SW
V
OUT1
V
IN
V
IN
LT3500
LDRV
BST
SW
V
OUT1
C3
V
IN
V
IN
D2
LT3500
LDRV
BST
SW
D2
V
OUT1
C3
3500 F05
The only reason to consider a larger diode is the worst-
case condition of a high input voltage and shorted output.
With a shorted condition, diode current will increase to a
typical value of 3A, determined by the peak switch current
limit of the LT3500. This is safe for short periods of time,
but it would be prudent to check with the diode manu-
facturer if continuous operation under these conditions
can be tolerated.
BST Pin Considerations
The capacitor and diode tied to the BST pin generate
a voltage that is higher than the input voltage. In most
cases a 0.47µF capacitor and fast switching diode (such
as the CMDSH-3 or FMMD914) will work well. Almost
any type of fi lm or ceramic capacitor is suitable, but the
ESR should be <1Ω to ensure it can be fully recharged
during the off time of the switch. The capacitor value can
be approximated by:
C
BST
=
I
OUT1(MAX)
•DC
50 • V
OUT1
− V
BST(MIN)
()
•f
where I
OUT1(MAX)
is the maximum load current, and
V
BST(MIN)
is the minimum boost voltage to fully saturate
the switch.
Figure 5 shows four ways to arrange the boost circuit.
The BST pin must be more than 2.2V above the SW pin
for full effi ciency.
Generally, for outputs of 3.3V and higher the standard
circuit (Figure 5a) is the best. For outputs between 2.8V
and 3.3V, replace the D2 with a small Schottky diode such
as the PMEG4005.
For lower output voltages the boost diode can be tied to
the input (Figure 5b). The circuit in Figure 5a is more ef-
fi cient because the BST pin current comes from a lower
voltage source.
Figure 5c shows the boost voltage source from the linear
output that is set to greater than 2.5V (any available DC
sources that are greater than 2.5V is suffi cient). The highest
effi ciency is attained by choosing the lowest boost volt-
age above 2.5V. You must also be sure that the maximum
voltage at the BST pin is less than the maximum specifi ed
in the Absolute Maximum Ratings section.
APPLICATIONS INFORMATION
LT3500
18
3500fc
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 2.5V, as
in Figure 5d. The diode is used to prevent damage to the
LT3500 in case V
X
is held low while V
IN
is present. The
circuit eliminates a capacitor, but effi ciency may be lower
and dissipation in the LT3500 may be higher. Also, if V
X
is
absent, the LT3500 will still attempt to regulate the output,
but will do so with very low effi ciency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
The minimum input voltage of an LT3500 application is
limited by the minimum operating voltage (<2.8V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT3500 is turned on with its SS pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
input and output voltages and on the arrangement of the
boost circuit.
The Typical Performance Characteristics section shows
plots of the minimum load current to start and to run as a
function of input voltage for 3.3V and 5V outputs. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
IN
is ramping very slowly.
Use a Schottky diode for the lowest start-up voltage.
Frequency Compensation
The LT3500 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3500 does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
C
pin. Generally a capacitor and a resistor in series to
ground determine loop gain. In addition, there is a lower
value capacitor in parallel. This capacitor is not part of
the loop compensation but is used to fi lter noise at the
switching frequency.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature.
The LT1375 data sheet contains a more thorough discus-
sion of loop compensation and describes how to test the
stability using a transient load.
Figure 6 shows an equivalent circuit for the LT3500 control
loop. The error amp is a transconductance amplifi er with
fi nite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance amplifi er generating an output
APPLICATIONS INFORMATION
Figure 6. Model for Loop Response
+
–
SW
LT3500
FB
V
C
0.8V
C
F
C
PL
V
OUT1
C1 C1
3500 F06
C
C
R
C
R1 ESR
TANTALUM
OR
POLYMER
CERAMIC
R2
4M
ERROR AMP
g
m
= 250µmhos
CURRENT MODE
POWER STAGE
g
m
= 3mho
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LT3500IMSE#TRPBF

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Buy LT3500IMSE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Monolithic 2A Step-Down Regulator plus Linear Regulator/Controller
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