Datasheet

LT3692A
17
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
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 per
-
formance. One benefit is that the larger capacitance may
give better transient response for large changes in load
current. Table 3 lists several capacitor vendors.
Table 3
VENDOR TYPE SERIES
Taiyo Yuden Ceramic X5R, X7R
AVX Ceramic X5R, X7R
Tantalum
Kemet Tantalum
T
A Organic
AL Organic
T491, T494, T495
T520
A700
Sanyo TA/AL Organic POSCAP
Panasonic AL Organic SP CAP
TDK Ceramic X5R, X7R
Catch Diode
The diode D1 conducts current only during switch-off
time. Use a Schottky diode to limit forward voltage drop to
increase efficiency. 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
OUT
V
IN
• V
IN
– V
OUT
( )
With a shorted condition, diode current will increase to the
typical value determined by the peak switch current limit
of the LT3692A set by the ILIM pin. This is safe for short
periods of time, but it would be prudent to check with the
diode manufacturer 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 a small Schottky diode (such as the
CMDSH-4E) will work well. To ensure optimal performance
at duty cycles greater than 80%, use a 0.5A Schottky
diode (such as a PMEG4005). Almost any type of film 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
OUT(MAX)
• V
OUT
5 • V
IN
V
OUT
– 2
( )
• f
where I
OUT(MAX)
is the maximum load current.
Figure 7 shows four ways to arrange the boost circuit. The
BST pin must be more than 3V above the SW pin for full
efficiency. Generally, for outputs of 3.3V and higher the
standard circuit (Figure 7a) is the best. For lower output
voltages the boost diode can be tied to the input (Fig
-
ure 7b). The circuit in Figure 7a is more efficient because
the BST pin current comes from a lower voltage source.
Figure 7c shows the boost voltage source from available
DC sources that are greater than 3V. The highest efficiency
is attained by choosing the lowest boost voltage above 3V.
For example, if you are generating 3.3V and 1.8V and the
3.3V is on whenever the 1.8V is on, the 1.8V boost diode
can be connected to the 3.3V output. In any case, you
must also be sure that the maximum voltage at the BST
pin is less than the maximum specified in the Absolute
Maximum Ratings section.
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 3V, as
in Figure 7d. The diode is used to prevent damage to the
LT3692A in case V
X
is held low while V
IN
is present. The
circuit saves several components (both BST pins can be
tied to D2). However, efficiency may be lower and dissipa
-
tion in the LT3692A may be higher. Also, if V
X
is absent,
the LT3692A will still attempt to regulate the output, but
will do so with very low efficiency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.