lt3503edcb-trpbf Linear Technology Corporation, lt3503edcb-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt3503edcb-trpbf
• Description: 1a, 2.2mhz Step-down Switching Regulator In 2mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Now the required on time has decreased below the mini-
mum on time of 130ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the switch pulse width remains fixed at
130ns. In Figure 2 the inductor current ramps up to a value
exceeding the load current and the output ripple increases
to ~40mV. The part then remains off until the output
voltage dips below 100% of the programmed value before
it begins switching again.
Provided that the output remains in regulation and that the
inductor does not saturate, operation above V
and will not damage the part. Figure 3 illustrates the
switching waveforms when the input voltage is increased
to its absolute maximum rating of 20V.
The part is robust enough to survive prolonged operation
under these conditions as long as the peak inductor
current does not exceed 2.2A. In Figure 3 the peak inductor
current of 2A suggests that the saturation current rating of
the inductor should be ~2.6A, which may require an
inductor of large physical size. The peak inductor current
value can be reduced by simultaneously increasing the
inductance and output capacitance. In Figure 4 the peak
inductor current is reduced to 1.3A by doubling the output
capacitor and inductor values. Now the required inductor
current saturation rating is ~1.7A, so that even though the
inductance value has increased, it may be possible to
achieve a physically smaller inductor size.
Note that inductor current saturation ratings often de-
crease with temperature and that inductor current satura-
tion may further limit performance in this operating regime.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 0.6 (V
OUT
+ V
U
D
)
U
W
IN(PS)
U
is safe
where V
L is in µH. With this value there will be no subharmonic
oscillation for applications with 50% or greater duty cycle.
The inductor’s RMS current rating must be greater than
your maximum load current and its saturation current
should be about 30% higher. For robust operation in fault
conditions, the saturation current should be above 2.2A.
To keep efficiency high, the series resistance (DCR) should
be less than 0.1Ω. Table 1 lists several vendors and types
that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
20mV/DIV
20mV/DIV
10V/DIV
10V/DIV
1A/DIV
1A/DIV
V
V
V
V
OUT
OUT
SW
SW
D
I
I
L
L
is the voltage drop of the catch diode (~0.4V) and
C
V
V
I
L = 1.1µH
C
V
V
I
L = 2.7µH
LOAD
LOAD
OUT
OUT
IN
OUT
OUT
IN
= 20V
= 20V
= 47µF
= 0.78V
= 2 × 47µF
= 0.78V
= 1.1A
= 1.1A
Figure 3
Figure 4
1µs/DIV
1µs/DIV
3503 F03
3503 F04
LT3503
9
3503f