Datasheet

LT1934/LT1934-1
8
1934fe
Which One to Use: LT1934 or LT1934-1?
The only difference between the LT1934 and LT1934-1
is the peak current through the internal switch and the
inductor. If your maximum load current is less than 60mA,
use the LT1934-1. If your maximum load is higher, use
the LT1934; it can supply up to ~300mA.
While the LT1934-1 can’t deliver as much output current,
it has other advantages. The lower peak switch current
allows the use of smaller components (input capacitor,
inductor and output capacitor). The ripple current at the
input of the LT1934-1 circuit will be smaller and may be
an important consideration if the input supply is current
limited or has high impedance. The LT1934-1’s current
draw during faults (output overload or short) and start-
up is lower.
The maximum load current that the LT1934 or LT1934-1
can deliver depends on the value of the inductor used.
Table 1 lists inductor value, minimum output capacitor
and maximum load for 3.3V and 5V circuits. Increasing
the value of the capacitor will lower the output voltage
ripple. Component selection is covered in more detail in
the following sections.
Minimum Input Voltage
The minimum input voltage required to generate a par-
ticular output voltage is determined by either the LT1934’s
undervoltage lockout of ~3V or by its maximum duty cycle.
APPLICATIONS INFORMATION
The duty cycle is the fraction of time that the internal
switch is on and is determined by the input and output
voltages:
DC = (V
OUT
+ V
D
)/(V
IN
– V
SW
+ V
D
)
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.3V at maximum load for the LT1934, ~0.1V for the
LT1934-1). This leads to a minimum input voltage of:
V
IN(MIN)
= (V
OUT
+ V
D
)/DC
MAX
– V
D
+ V
SW
with DC
MAX
= 0.85.
Inductor Selection
A good fi rst choice for the inductor value is:
L = 2.5 • (V
OUT
+ V
D
) • 1.8μs/I
LIM
where I
LIM
is the switch current limit (400mA for the
LT1934 and 120mA for the LT1934-1). This choice provides
a worst-case maximum load current of 250mA (60mA for
the LT1934-1). The inductor’s RMS current rating must
be greater than the load current and its saturation current
should be greater than I
LIM
. To keep effi ciency high, the
series resistance (DCR) should be less than 0.3Ω (1Ω
for the LT1934-1). Table 2 lists several vendors and types
that are suitable.
This simple rule may not provide the optimum value for
your application. If the load current is less, then you can
relax the value of the inductor and operate with higher
ripple current. This allows you to use a physically smaller
inductor, or one with a lower DCR resulting in higher
effi ciency. The following provides more details to guide
inductor selection. First, the value must be chosen so that
the LT1934 can supply the maximum load current drawn
from the output. Second, the inductor must be rated ap-
propriately so that the LT1934 will function reliably and
the inductor itself will not be overly stressed.
Detailed Inductor Selection and
Maximum Load Current
The square wave that the LT1934 produces at its switch
pin results in a triangle wave of current in the inductor. The
LT1934 limits the peak inductor current to I
LIM
. Because
Table 1
PART V
OUT
L
MINIMUM
C
OUT
MAXIMUM
LOAD
LT1934 3.3V 100μH
47μH
33μH
100μH
47μH
33μH
300mA
250mA
200mA
5V 150μH
68μH
47μH
47μH
33μH
22μH
300mA
250mA
200mA
LT1934-1 3.3V 150μH
100μH
68μH
15μH
10μH
10μH
60mA
45mA
20mA
5V 220μH
150μH
100μH
10μH
4.7μH
4.7μH
60mA
45mA
20mA