Datasheet

LT3988
12
3988f
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3988 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes. Because
loop stability and transient response depend on the value
of C
OUT
, this loss may be unacceptable. Use X7R and X5R
types.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Tantalum, as well as newer,
lower-ESR organic electrolytic capacitors intended for
power supply use are suitable. Choose a capacitor with a
low enough ESR for the required output 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 similar ripple performance. One benefit
is that the larger capacitance may give better transient
response for large changes in load current. Table 2 lists
several capacitor vendors.
Table 2. Low ESR Surface Mount Capacitors
MFG TYPE SERIES
AVX Ceramic
Tantalum TPS
Johansen Ceramic X7R, 1812 MLCC
Kemet Tantalum
Tantalum Organic
Aluminum Organic
T491,T494,T498
T520,T521,T528
A700
Panasonic Aluminum Organic SP CAP
Sanyo Tantalum
Aluminum Organic
POSCAP
Taiyo-Yuden Ceramic
TDK Ceramic
Diode Selection
The catch diode (D1 from Figure 1) conducts the inductor
current during the switch off time. Use a Schottky diode
rated for 1A to 2A average current. Peak reverse voltage
across the diode is equal to the regulator input voltage.
Use a diode with a reverse voltage rating greater than the
input voltage. The OVLO function of the LT3988 turns off
the switch when V
IN
> 64V (typ) allowing use of Schottky
applicaTions inForMaTion
diodes with a 70V rating for input voltages up to 80V. Table3
lists several Schottky diodes and their manufacturers.
Table 3. Schottky Diodes
PART NUMBER
V
R
(V)
I
AVG
(A)
V
F
AT 1A
(mV)
V
F
AT 2A
(mV)
On Semiconductor
NSR10F40NXT5G 40 1 490
MBRA160T3 60 1 510
MBRS190T3 90 1 750
MBRS260T3G 60 2 430
Diodes Inc
B140 40 1 500
B160 60 1 700
B170 70 1 790
B180 80 1 790
B260 60 2 700
B280 80 2 790
DFLS140L 40 1 550
DFLS160L 60 1 500
DFLS260 60 2 620
Boost Pin Considerations
The external capacitor and the internal diode tied to the
BOOST pin generate a voltage that is higher than the input
voltage. In most cases, a small ceramic capacitor will work
well. The capacitor value is a function of the switching
frequency, peak current, duty cycle and boost voltage.
Figure 2 shows three ways to arrange the boost circuit. The
BOOST pin must be more than 2.3V above the SW pin for
full efficiency. For outputs of 3.3V and higher, the standard
circuit (Figure 2a) is best. For lower output voltages, the BD
pin can be tied to the input (Figure 2b). The circuit in Figure
2a is more efficient because the BOOST pin current comes
from a lower voltage source. Finally, as shown in Figure
2c, the BD pin can be tied to another source that is at least
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 BD pin can be
connected to the 3.3V output. (see Output Voltage Tracking).
Be sure that the maximum voltage at the BOOST pin is less
than 80V and the voltage difference between the BOOST
and SW pins is less than 30V. The minimum operating
voltage of an LT3988 application is limited by the internal
4V undervoltage lockout and by the maximum duty cycle.