LT1934-1 Linear Technology, LT1934-1 Datasheet - Page 7

• Manufacturer Part Number: LT1934-1
• Description: Micropower Step-Down Switching Regulators in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 particu-
lar output voltage is determined by either the LT1934’s
undervoltage lockout of ~3V or by its maximum duty
Table 1
PART
LT1934
LT1934-1
V
3.3V
3.3V
5V
5V
OUT
U
100 H
150 H
150 H
100 H
220 H
150 H
100 H
47 H
33 H
68 H
47 H
68 H
L
U
MINIMUM
100 F
4.7 F
4.7 F
C
47 F
33 F
47 F
33 F
22 F
15 F
10 F
10 F
10 F
W
OUT
MAXIMUM
U
300mA
250mA
200mA
300mA
250mA
200mA
LOAD
60mA
45mA
20mA
60mA
45mA
20mA
cycle. The duty cycle is the fraction of time that the internal
switch is on and is determined by the input and output
voltages:
where V
(~0.4V) and V
(~0.3V at maximum load for the LT1934, ~0.1V for the
LT1934-1). This leads to a minimum input voltage of:
with DC
Inductor Selection
A good first choice for the inductor value is:
where I
LT1934 and 120mA for the LT1934-1). This choice pro-
vides 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
efficiency high, the series resistance (DCR) should be less
than 0.3 (1
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
efficiency. 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
appropriately 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
DC = (V
V
L = 2.5 • (V
IN(MIN)
MAX
LIM
D
is the forward voltage drop of the catch diode
OUT
= (V
is the switch current limit (400mA for the
= 0.85.
SW
OUT
+ V
OUT
for the LT1934-1). Table 2 lists several
is the voltage drop of the internal switch
D
+ V
)/(V
+ V
D
) • 1.8 s/I
IN
D
)/DC
– V
LT1934/LT1934-1
SW
MAX
+ V
www.DataSheet4U.com
LIM
– V
D
D
)
+ V
SW
LIM
LIM
. Because
. To keep
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