LT1109_15 Datasheet, PDF (5/8 Pages) Linear Technology – Micropower Low Cost DC/DC Converter Adjustable and Fixed 5V, 12V

LT110 9
LT1109 S8 A D 8 OPERATIO
The 8-pin versions of the LT1109 have separate pins for
V_IN and SENSE or FB and also have a SHUTDOWN pin.
Separating the device V_IN pin from the SENSE pin allows
the device to be powered from the (lower) input voltage
rather than the (higher) output voltage. Although quies-
cent current remains constant, quiescent power will be
reduced by using the 8-pin version since the quiescent
current flows from a lower voltage source. The SHUT-
DOWN pin disables the oscillator when taken to a logic “0.”
If left floating or tied high, the converter operates nor-
mally. With SHUTDOWN low, quiescent current remains
at 320µA.
APPLICATI S I FOR ATIO
Inductor Selection
A DC/DC converter operates by storing energy as mag-
netic flux in an inductor core, and then switching this
energy into the load. To operate as an efficient energy
transfer element, the inductor must fulfill three require-
ments. First, the inductance must be low enough for the
inductor to store adequate energy under the worst case
condition of minimum input voltage and switch-ON time.
The inductance must also be high enough so that maxi-
mum current ratings of the LT1109 and inductor are not
exceeded at the other worst case condition of maximum
input voltage and ON time. Additionally, the inductor core
must be able to store the required flux; i.e., it must not
saturate. At power levels generally encountered with
LT1109 designs, small ferrite surface-mount inductors
will function well. Lastly, the inductor must have suffi-
ciently low DC resistance so that excessive power is not
lost as heat in the windings. Look for DCR values in the
inductors’ specification tables; values under 0.5Ω will give
best efficiency. An additional consideration is Electro-
Magnetic Interference (EMI). Toroid and pot core type
inductors are recommended in applications where EMI
must be kept to a minimum; for example, where there are
sensitive analog circuitry or transducers nearby. Rod core
types are a less expensive choice where EMI is not a
problem.
Specifying a proper inductor for an application requires
first establishing minimum and maximum input voltage,
output voltage, and output current. In a step-up converter,
the inductive events add to the input voltage to produce the
output voltage. Power required from the inductor is deter-
mined by
P_L = (V_OUT + V_D – V_IN) (I_OUT)
(01)
where V_D is the diode drop (0.5V for a 1N5818 Schottky).
Energy required by the inductor per cycle must be equal or
greater than
P_L
F_OSC
(⁰²)
in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds
according to
( ) ^I_L
t
=
V_IN
R'

^{1 –}
–R't _
e _{L }
(⁰³)
where R' is the sum of the switch equivalent resistance
(0.8 typical at 25°C) and the inductor DC resistance. When
the drop across the switch is small compared to V_IN, the
simple lossless equation
( ) ^I_L
t
= ^VIN t
L
(⁰⁴)
can be used. These equations assume that at t = 0,
inductor current is zero. This situation is called “discon-
tinuous mode operation” in switching regulator parlance.
Setting “t” to the switch-ON time from the LT1109 speci-
fication table (typically 4.2µs) will yield I_PEAK for a specific
“L” and V_IN. Once I_PEAK is known, energy in the inductor
at the end of the switch-ON time can be calculated as
E_L
=
1
2
LI_P2EAK
(⁰⁵)
E_L must be greater than P_L/F_OSC for the converter to deliver
the required power. For best efficiency I_PEAK should be
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