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LT3688IUF#TRPBF

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LT3688
16
3688f
APPLICATIONS INFORMATION
additional bulk capacitance may be necessary. This can be
provided with a lower performance electrolytic capacitor.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3688 input and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 2.2F capacitor is capable of this task, but only if it is
placed close to the LT3688 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3688. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3688 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3688’s
voltage rating. See Linear Technology’s Application Note
88 for details.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT3688 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3688’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
50
V
OUT
•f
SW
where f
SW
is in MHz and C
OUT
is the recommended output
capacitance in F. Use X5R or X7R types, which 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 benefi t (see the
Compensation section).
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 specifi ed by the supplier and should be 0.1
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 3 lists
several capacitor vendors.
Table 3. Capacitor Vendors
VENDOR PART SERIES COMMENTS
Panasonic Ceramic
Polymer
Tantalum
EEEF Series
Kemet Ceramic
Tantalum T494, T495
Sanyo Ceramic
Polymer
Tantalum
POSCAP
Murata Ceramic
AVX Ceramic
Tantalum TPS Series
Taiyo Yuden Ceramic
Catch Diode
The catch diode conducts current only during switch-off
time. Average forward current in normal operation can
be calculated from:
I
D(AVG)
=
I
OUT
V
IN
–V
OUT
()
V
IN
where I
OUT
is the output load current. The only reason to
consider a diode with a larger current rating than neces-
sary for nominal operation is for the worst-case condition
of shorted output. The diode current will then increase to
the typical peak switch current limit. Peak reverse voltage
is equal to the regulator input voltage. Use a Schottky
diode with a reverse voltage rating greater than the input
voltage. Table 4 lists several Schottky diodes and their
manufacturers.
LT3688
17
3688f
Table 4. Capacitor Vendors
Part Number
V
R
(V)
I
AVE
(A)
V
F
at 1A
(mV)
On Semiconductor
MBR0520L 20 0.5
MBR0540 40 0.5 620
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B0530W 30 0.5
B120 20 1 500
B130 30 1 500
B140HB 40 1
DFLS140 40 1.1 510
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3688 due to their piezoelectric
nature. When in Burst Mode operation, the LT3688’s
switching frequency depends on the load current, and
at very light loads the LT3688 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3688 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
Frequency Compensation
The LT3688 uses current mode control to regulate the
output, which simplifi es loop compensation. In particular,
the LT3688 does not require the ESR of the output capaci-
tor for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. Figure 3
shows an equivalent circuit for the LT3688 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 transconduc-
tance amplifi er generating an output current proportional to
the voltage at the V
C
node. Note that the output capacitor,
C1, integrates this current, and that the capacitor on the
V
C
node (C
C
) integrates the error amplifi er output current,
resulting in two poles in the loop. R
C
provides a zero.
With the recommended output capacitor, the loop cross-
over occurs above the R
C
C
C
zero. This simple model works
well as long as the value of the inductor is not too high
and the loop crossover frequency is much lower than the
switching frequency. With a larger ceramic capacitor (very
low ESR), crossover may be lower and a phase lead ca-
pacitor (C
PL
) across the feedback divider may improve the
phase margin and transient response. At minimum, use a
10pF phase lead capacitor to reduce noise injection to the
FB pin. If the output capacitor is different than the recom-
mended capacitor, stability should be checked across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Figure 4
shows the transient response when the load current is
stepped from 300mA to 600mA and back to 300mA.
–
+
800mV
V
C
LT3688
GND
3688 F03
R1
OUT
ESR
TANTALUM OR
ELECTROLYTIC
CERAMIC
ERROR
AMPLIFIER
CURRENT MODE
POWER ST
AGE
FB
R2
3M
R
C
80k
C
C
100pF
C1
C1
g
m
=
1.6A/V
+
C
PL
0.7V
–
+
g
m
=
300µA/V
Figure 3. Model for the Loop Response
Figure 4. Transient Load Response of the LT3688
Front Page Application as the Load Current is
Stepped from 300mA to 600mA
APPLICATIONS INFORMATION
V
OUT
100mV/DIV
I
LOAD
200mA/DIV
50µs/DIV
3688 F04
LT3688
18
3688f
Low Ripple Burst Mode Operation
To enhance effi ciency at light loads, the LT3688 operates
in low ripple Burst Mode operation that keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera-
tion, the LT3688 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3688 delivers power to the
output with single, low current pulses, the output ripple
is kept below 25mV for a typical application. In addition,
V
IN
and BIAS quiescent currents are reduced to typically
65A and 155A, respectively, during the sleep time. As
the load current decreases towards a no-load condition,
the percentage of time that the LT3688 operates in sleep
mode increases and the average input current is greatly
reduced, resulting in high effi ciency even at very low loads
(see Figure 5). At higher output loads the LT3688 will be
running at the frequency programmed by the R
T
resistor,
and will be operating in standard PWM mode. The transi-
tion between PWM and low ripple Burst Mode operation
is seamless, and will not disturb the output voltage. The
front page application circuit will switch at full frequency
at output loads higher than about 60mA.
I
L
0.2A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5µs/DIV
3688 F05
Figure 5. Burst Mode Operation
BST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diodes (see
the Block Diagram) are used to generate boost voltages
that are higher than the input voltage. In most cases, a
0.22F capacitor will work well. For the best performance
in dropout, use a 1µF or larger capacitor. Figure 6 shows
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
GND
LT3688
(6a) For V
OUT
> 2.8V
Figure 6. Three Circuits for Generating the Boost Voltage
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
D2
GND
LT3688
(6b) For 2.5V < V
OUT
< 2.8V
V
IN
BST
SW
BIAS
V
IN
V
OUT
4.7µF
C3
GND
LT3688
3688 F06
(6c) For V
OUT
< 2.5V; V
IN(MAX)
= 30V
APPLICATIONS INFORMATION
three ways to arrange the boost circuit. The BST pin must
be more than 2.3V above the SW pin for best effi ciency.
For outputs of 3V and above, the standard circuit (Figure
6a) is best. For outputs between 2.8V and 3V, use a 1F
boost capacitor. A 2.5V output presents a special case
because it is marginally adequate to support the boosted
drive stage while using the internal boost diode. For reliable
BST pin operation with 2.5V outputs, use a good external
Schottky diode (such as the ON semi MBR0540), and a
1F boost capacitor (see Figure 6b). For lower output
voltages, the boost diode can be tied to the input (Figure
6c), or to another supply greater than 2.8V. The circuit in
Figure 6a is more effi cient because the BST pin current
and BIAS pin quiescent current comes from a lower volt-
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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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