Datasheet

LT3844
18
3844fb
APPLICATIONS INFORMATION
Minimum On-Time Considerations (Buck Mode)
Minimum on-time, t
ON(MIN)
, is the smallest amount of time
that the LT3844 is capable of turning the top MOSFET on
and off again. It is determined by internal timing delays
and the amount of gate charge required turning on the
top MOSFET. Low duty cycle applications may approach
this minimum on-time limit and care should be taken to
ensure that:
t
V
Vf
t
ON
OUT
IN SW
ON MIN
=>
•
()
where t
ON(MIN)
is typically 350ns worst case.
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LT3844 will begin to skip cycles.
The output will be regulated, but the ripple current and
ripple voltage will increase. If lower frequency operation
is acceptable, the on-time can be increased above t
ON(MIN)
for the same step-down ratio.
Boost Converter Design
The LT3844 can be used to confi gure a boost converter
to step-up voltages to as high as hundreds of volts. An
example of a boost converter circuit schematic is shown
in the Typical Applications section. The following sections
are a guide to designing a boost converter:
The maximum duty cycle of the main switch is:
DC
VV
V
MAX
OUT IN MIN
OUT
=
−
()
Boost Converter: Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value and saturation current. The
minimum inductance value is calculated as follows:
L
V
If
DC
MIN
IN MIN
LSW
MAX
=
()
•
•
Δ
f
SW
is the switch frequency.
Similar to the buck converter, the typical range of values
for ΔI
L
is (0.2 • I
L(MAX)
) to (0.5 • I
L(MAX)
), where I
L(MAX)
is the maximum average inductor current.
II
V
V
L MAX OUT MAX
OUT
IN MIN
() ()
()
•=
Using ΔI
L
= 0.3 • I
L(MAX)
yields a good design compromise
between inductor performance versus inductor size and
cost.
The inductor must not saturate at the peak operating
current, I
L(MAX)
+ ΔI
L
/2. The inductor saturation current
specifi cation is the current at which the inductance, mea-
sured at zero current, decreases by a specifi ed amount,
typically 30%.
One drawback of boost regulators is that they cannot be
current limited for output shorts because the current steer-
ing diode makes a direct connection between input and
output. Therefore, the inductor current during an output
short circuit is only limited by the available current of the
input supply.
After calculating the minimum inductance value and the
saturation current for your design, select an off-the-shelf
inductor. For more detailed information on selecting an
inductor, please see the “Inductor Selection” section of
Linear Technology Application Note 19.
Boost Converter: MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFET include on resistance (R
DS(ON)
), re-
verse transfer capacitance (C
RSS
), maximum drain source
voltage (V
DSS
), total gate charge (Q
G
) and maximum
continuous drain current.
For maximum effi ciency, minimize R
DS(ON)
and C
RSS
.
Low R
DS(ON)
minimizes conduction losses while low
C
RSS
minimizes transition losses. The problem is that
R
DS(ON)
is inversely related to C
RSS
. Balancing the tran-
sition losses with the conduction losses is a good idea
in sizing the MOSFET. Select the MOSFET to balance the