LT3992EFE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3992EFE' PDF : 36 Pages View PDF
LT3992
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
Taiyo Yuden
AVX
Kemet
Sanyo
Panasonic
TDK
TYPE
Ceramic X5R, X7R
Ceramic X5R, X7R
Tantalum
Tantalum
TA Organic
AL Organic
TA/AL Organic
AL Organic
Ceramic X5R, X7R
SERIES
T491, T494, T495
T520
A700
POSCAP
SP CAP
Catch Diode
The diode D1 (Figure 1) conducts current only during
switch-off time. Use a Schottky diode to limit forward volt-
age 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:
( ) ID(AVG)
=
IOUT
VIN
•
VIN – VOUT
With a shorted condition, diode current will increase to the
typical value determined by the peak switch current limit
of the LT3992 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
BAT41) will work well. To ensure optimal performance at
duty cycles greater than 80%, use a 0.5A Schottky diode
(such as a MBR0560). 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:
( ) CBST
=
IOUT(MAX) •
5 • VIN VOUT
VOUT
–2 •
f
where IOUT(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
LT3992 in case VX is held low while VIN 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 LT3992 may be higher. Also, if VX is absent, the
LT3992 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.
3992fa
16
For more information www.linear.com/LT3992
Share Link: 
