lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 11

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A
larger value inductor provides a slightly higher maximum
load current and will reduce the output voltage ripple. If
your load is lower than the maximum load current, 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. Be aware that if the inductance differs
from the simple rule above, then the maximum load
current will depend on input voltage. In addition, low
inductance may result in discontinuous mode operation,
which further reduces maximum load current. For details
of maximum output current and discontinuous mode
operation, see Linear Technology’s Application Note AN44.
Finally, for duty cycles greater than 50% (V
a minimum inductance is required to avoid subharmonic
oscillations. See AN19.
The current in the inductor is a triangle wave with an
average value equal to the load current. The peak switch
current is equal to the output current plus half the peak-to-
peak inductor ripple current. The LT1941 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT1941 will deliver depends on the switch current
limit, the inductor value and the input and output voltages.
When the switch is off, the potential across the inductor is
the output voltage plus the catch diode drop. This gives the
peak-to-peak ripple current in the inductor:
where f is the switching frequency of the LT1941 and L is
the value of the inductor. The peak inductor and switch
current is:
To maintain output regulation, this peak current must be
less than the LT1941’s switch current limit I
I
LIM
∆I
I
SWPK
is at least 3A at low duty cycles and decreases linearly
L
= (1 – DC)(V
= I
LPK
= I
OUT
U
OUT
+ ∆I
+ V
U
L
F
)/(L • f)
/2
W
OUT
LIM
/V
. For SW1,
U
IN
> 0.5),
to 2.4A at DC = 0.8. For SW2, I
duty cycles and decreases linearly to 1.6A at DC = 0.8. The
maximum output current is a function of the chosen
inductor value:
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors
and choose one to meet cost or space goals. Then use
these equations to check that the LT1941 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when I
∆I
Output Capacitor Selection
For 5V and 3.3V outputs, a 10µF, 6.3V ceramic capacitor
(X5R or X7R) at the output results in very low output
voltage ripple and good transient response. For lower
voltages, 10µF is adequate for ripple requirements but
increasing C
types and values will also work; the following discusses
tradeoffs in output ripple and transient performance.
The output capacitor filters the inductor current to gener-
ate an output with low voltage ripple. It also stores energy
in order to satisfy transient loads and stabilize the LT1941’s
control loop. Because the LT1941 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without the
presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
L
I
OUT(MAX)
/2.
OUT
= I
= 3 • (1 – 0.25 • DC) – ∆I
= 2 • (1 – 0.25 • DC) – ∆I
LIM
will improve transient performance. Other
– ∆I
L
/2
LIM
is at least 2A for at low
L
L
/2 for SW1
/2 for SW2
OUT
LT1941
is less than
11
1941fa