LT3501 LINER [Linear Technology], LT3501 Datasheet - Page 16

• Manufacturer Part Number: LT3501
• Description: Monolithic Dual Tracking 3A Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3501
Table 2
Catch Diode
The diode D1 conducts current only during switch off
time. Use a Schottky diode to limit forward voltage drop to
increase effi ciency. 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:
The only reason to consider a larger diode is the worst-
case condition of a high input voltage and shorted output.
With a shorted condition, diode current will increase to a
typical value of 4A, determined by the peak switch current
limit of the LT3501. This is safe for short periods of time,
but it would be prudent to check with the diode manu-
facturer 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 fast switching diode (such
as the CMDSH-3 or FMMD914) will work well. Almost
any type of fi lm 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:
16
I
C
D AVG
Taiyo Yuden
BST
(
Panasonic
VENDOR
Kemet
Sanyo
TDK
AVX
=
)
=
B
I
•
OUT
V
(
IN
V
I
OUT
OUT MAX
•
(
V
(
–
Ceramic X5R, X7R
Ceramic X5R, X7R
Ceramic X5R, X7R
IN
TA/AL Organic
V
AL Organic
AL Organic
–
BST MIN
TA Organic
Tantalum
Tantalum
)
V
TYPE
•
OUT
(
DC
)
)
)
•
f
T491, T494, T495
POSCAP
SERIES
SP CAP
T520
A700
where I
V
the switch.
Figure 5 shows four ways to arrange the boost circuit. The
BST pin must be more than 1.4V above the SW pin for
full effi ciency. Generally, for outputs of 3.3V and higher
the standard circuit (Figure 5a) is the best. For outputs
between 2.8V and 3.3V, replace the D2 with a small
Schottky diode such as the PMEG4005. For lower output
voltages the boost diode can be tied to the input (Figure
5b). The circuit in Figure 5a is more effi cient because the
BST pin current comes from a lower voltage source. Fig-
ure 5c shows the boost voltage source from available DC
sources that are greater than 3V. The highest effi ciency 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 specifi ed 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 5d. The diode is used to prevent damage to the
LT3501 in case V
circuit saves several components (both BST pins can be
tied to D2). However, effi ciency may be lower and dissipa-
tion in the LT3501 may be higher. Also, if V
LT3501 will still attempt to regulate the output, but will do
so with very low effi ciency and high dissipation because
the switch will not be able to saturate, dropping 1.5V to
2V in conduction.
The minimum input voltage of an LT3501 application is
limited by the minimum operating voltage (<3V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT3501 is turned on with its SS pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
BST(MIN)
OUT(MAX)
is the minimum boost voltage to fully saturate
X
is the maximum load current, and
is held low while V
IN
is present. The
X
is absent, the
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