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LT3688IUF#TRPBF

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LT3688
13
3688f
APPLICATIONS INFORMATION
Setting the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors
according to:
R1= R2
V
OUT
0.8V
–1
⎛
⎝
⎜
⎞
⎠
⎟
For reference designators, refer to the Block Diagram.
Setting the Switching Frequency
The LT3688 uses a constant-frequency PWM architecture
that can be programmed to switch from 350 kHz to 2.2 MHz
by using a resistor tied from the RT pin to ground. Table 1
shows the R
T
values for various switching frequencies
Table 1. Switching Frequency vs R
T
SWITCHING FREQUENCY (MHz)
R
T
(kΩ)
0.35 165
0.5 110
0.6 88.7
0.7 75
0.8 64.9
0.9 56.2
1 49.9
1.2 40.2
1.4 33.2
1.6 27.4
1.8 23.2
2.1 20
2.3 17.4
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
effi ciency, component size and maximum input voltage.
The advantage of high frequency operation is that
smaller inductor and capacitor values may be used. The
disadvantages are lower effi ciency, and narrower input
voltage range at constant-frequency. The highest constant-
switching frequency (f
SW(MAX)
) for a given application can
be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
F
t
ON(MIN)
V
IN
+ V
F
–V
SW
()
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
F
is the catch diode drop (~0.5V) and V
SW
is
the internal switch drop (~0.3V at maximum load). If the
LT3688 is programmed to operate at a frequency higher
than f
SW(MAX)
for a given input voltage, the LT3688 enters
pulse skip mode, where it skips switching cycles to maintain
regulation. At frequencies higher than f
SW(MAX)
, the LT3688
no longer operates with constant frequency. The LT3688
enters pulse skip mode at frequencies higher than f
SW(MAX)
because of the limitation on the LT3688’s minimum on time
of 140ns (180ns for T
J
> 125°C). As the switching frequency
is increased above f
SW(MAX)
, the part is required to switch
for shorter periods to maintain the same duty cycle. Delays
associated with turning off the power switch dictate the
minimum on-time of the part. When the required on-time
decreases below the minimum on-time of 140ns, the switch
pulse width remains fi xed at 140ns (instead of becoming
narrower) to accommodate the same duty cycle require-
ment. The inductor current ramps up to a value exceeding
the load current and the output ripple increases. The part
then remains off until the output voltage dips below the
programmed value before it begins switching again.
Maximum Operating Voltage Range
The maximum input voltage for LT3688 applications
depends on switching frequency, the absolute maximum
ratings of the V
IN
and BST pins, and by the minimum
duty cycle (DC
MIN
). The LT3688 can operate from input
voltages up to 36V.
DC
MIN
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is equal to 140ns and f
SW
is the switching
frequency. Running at a lower switching frequency allows
a lower minimum duty cycle. The maximum input voltage
before pulse-skipping occurs depends on the output volt-
age and the minimum duty cycle:
V
IN(PS)
=
V
OUT
+ V
F
DC
MIN
–V
F
+ V
SW
Example: f = 2.1MHz, V
OUT
= 3.3V
DC
MIN
= 140ns • 2.1MHz = 0.294
V
IN(PS)
=
3.3V + 0.5V
0.294
– 0.5V + 0.3V = 12.7V
LT3688
14
3688f
APPLICATIONS INFORMATION
The LT3688 will regulate the output voltage at input volt-
ages greater than V
IN(PS)
. For example, an application
with an output voltage of 3.3V and switching frequency
of 2.1MHz has a V
IN(PS)
of 12.7V, as shown in Figure 1.
Figure 2 shows operation at 27V. Output ripple and peak
inductor current have signifi cantly increased. A saturating
inductor may further reduce performance. In pulse skip
mode, the LT3688 skips switching pulses to maintain
output regulation. The LT3688 will also skip pulses at very
low load currents. V
IN(PS)
vs load current is plotted in the
Typical Performance section.
V
OUT
50mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3688 F01
Figure 1. Operation Below Pulse-Skipping
Voltage. V
OUT
= 3.3V and f
SW
= 2.1MHz
V
OUT
50mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3688 F02
Figure 2. Operation Above V
IN(ps)
. V
IN
= 27V,
V
OUT
= 3.3V and f
SW
= 2.1MHz. Output Ripple
and Peak Inductor Current Increase
Minimum Operating Voltage Range
The minimum input voltage is determined either by the
LT3688’s minimum operating voltage of ~3.6V 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
F
V
IN
–V
SW
+ V
F
Unlike many fi xed frequency regulators, the LT3688 can
extend its duty cycle by remaining on for multiple cycles.
The LT3688 will not switch off at the end of each clock
cycle if there is suffi cient voltage across the boost capacitor
(C3 in the Block Diagram). Eventually, the voltage on the
boost capacitor falls and requires refreshing. Circuitry
detects this condition and forces the switch to turn off,
allowing the inductor current to charge up the boost
capacitor. This places a limitation on the maximum duty
cycle as follows:
DC
MAX
= 90%
This leads to a minimum input voltage of:
V
IN(MIN)
=
V
OUT
+ V
F
DC
MAX
–V
F
+ V
SW
where V
F
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.3V at maximum load).
Example: I
SW
=0.8A and V
OUT
= 3.3V
V
IN(MIN)
=
3.3V + 0.4V
90%
– 0.4 + 0.3V = 4V
For best performance in dropout, use a 1µF or larger
boost capacitor.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is
L = V
OUT
+ V
F
()
•
1.8MHz
f
SW
where V
F
is the voltage drop of the catch diode (~0.4V),
f
SW
is in MHz, and L is in H. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be at least 30% higher.
For robust operation in fault conditions (start-up or short-
circuit) and high input voltage (>30V), use an 8.2µH or
greater inductor with a saturation rating of 2.2A, or higher.
For highest effi ciency, the series resistance (DCR) should
be less than 0.1. Table 2 lists several vendors and types
that are suitable.
LT3688
15
3688f
APPLICATIONS INFORMATION
Table 2. Inductor Vendors
VENDOR PART SERIES TYPE URL
Murata LQH55D Open www.murata.com
TDK SLF7045
SLF10145
Shielded
Shielded
www.component.tdk.com
Toko DC62CB
D63CB
D75C
D75F
Shielded
Shielded
Shielded
Open
www.toko.com
Sumida CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
www.sumida.com
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current,
and reduces the output voltage ripple. If your load is lower
than the maximum load current, then you can relax the
value of the inductor and operate with higher ripple current.
This allows you to use a physically smaller inductor, or
one with a lower DCR resulting in higher effi ciency. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
input voltage. In addition, low inductance may result in
discontinuous mode operation, which further reduces
maximum load current. Discontinuous operation occurs
when I
OUT
is less than ∆I
L
/ 2. For details of maximum
output current and discontinuous mode operation, see
Linear Technology’s Application Note AN44. Finally, for
duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), a minimum
inductance is required to avoid sub-harmonic oscillations:
L
MIN
= V
OUT
+ V
F
()
•
1.2MHz
f
SW
where V
F
is the voltage drop of the catch diode (~0.4V),
f
SW
is in MHz, and L
MIN
is in H.
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3688 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3688 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the induc-
tor is the output voltage plus the catch diode drop. This
gives the peak-to-peak ripple current in the inductor
∆I
L
=
1–DC
()
V
OUT
+ V
F
()
L•f
where f is the switching frequency of the LT3688 and L is the
value of the inductor. The peak inductor and switch current is
I
SW(PK)
= I
L(PK)
= I
OUT
+
∆I
L
2
To maintain output regulation, this peak current must be
less than the LT3688’s switch current limit I
LIM
. I
LIM
is at
least 1.25A for at low duty cycles and decreases linearly
to 0.9A at DC = 0.9. The maximum output current is a
function of the chosen inductor value:
I
OUT(MAX)
= I
LIM
–
∆I
L
2
= 1.25A • 1– 0.3DC
()
–
∆I
L
2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors, and
choose one to meet cost or space goals. Then use these
equations to check that the LT3688 will be able to deliver
the required output current. Note again that these equations
assume that the inductor current is continuous.
Input Capacitor
Bypass the input of the LT3688 circuit with a ceramic
capacitor of an X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 2.2F to 4.7F ceramic capacitor
is adequate to bypass the LT3688 and will easily handle
the ripple current. Note that larger input capacitance
is required when a lower switching frequency is used.
If the input power source has high impedance, or there
is signifi cant inductance due to long wires or cables,
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LT3688IUF#TRPBF

Mfr. #:
Buy LT3688IUF#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual 800mA Step-Down Switching Regulator with Power-On Reset and Watchdog Timer
Lifecycle:
New from this manufacturer.
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