lt3503edcb-trpbf Linear Technology Corporation, lt3503edcb-trpbf Datasheet - Page 8

• 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
LT3503
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1%
resistors according to:
R2 should be 20.0k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
An optional phase lead capacitor of 22pF between V
and FB reduces light-load output ripple.
Input Voltage Range
The input voltage range for LT3503 applications depends
on the output voltage and on the absolute maximum
ratings of the V
The minimum input voltage is determined by either the
LT3503’s minimum operating voltage of 3.6V, or by its
maximum duty 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
(~0.4V at maximum load). This leads to a minimum input
voltage of:
with DC
The maximum input voltage is determined by the absolute
maximum ratings of the V
stant-frequency operation the maximum input voltage is
determined by the minimum duty cycle, DC
the duty cycle requirement is less than DC
enter pulse-skipping mode. The onset of pulse-skipping
occurs at:
8
R
DC
V
V
IN MIN
1
IN PS
=
(
(
=
MAX
R
D
V
2
)
IN
is the forward voltage drop of the catch diode
)
=
⎛
⎝ ⎜
V
=
= 0.81 (0.76 over temperature).
0 78
OUT
–
V
V
SW
.
V
OUT
V
OUT
DC
IN
OUT
DC
SW
is the voltage drop of the internal switch
+
V
and BOOST pins.
MAX
MIN
V
U
+
+
+
–
D
V
1
V
V
⎞
⎠ ⎟
D
D
D
–
–
U
IN
V
V
D
D
and BOOST pins. For con-
+
+
V
V
SW
SW
W
MIN
MIN
, the part will
U
= 0.29. If
OUT
In pulse-skipping mode the part skips pulses to control the
inductor current and regulate the output voltage, possibly
producing a spectrum of frequencies below 2.2MHz.
Note that this is a restriction on the operating input voltage
to remain in constant-frequency operation; the circuit will
tolerate transient inputs up to the absolute maximum
ratings of the V
regulation. The input voltage should be limited to V
during overload conditions (short-circuit or start-up).
Minimum On Time
The part will still regulate the output at input voltages that
exceed V
increases. Figure 1 illustrates switching waveforms in
continuous mode for a 0.78V output application near
V
As the input voltage is increased, the part is required to
switch for shorter periods of time. Delays associated with
turning off the power switch dictate the minimum on time
of the part. The minimum on time for the LT3503 is
~130ns. Figure 2 illustrates the switching waveforms
when the input voltage is increased to V
IN(PS)
20mV/DIV
20mV/DIV
10V/DIV
10V/DIV
1A/DIV
1A/DIV
V
V
V
V
= 6V.
OUT
OUT
SW
SW
I
I
IN(PS)
L
L
C
V
V
I
L = 1.1µH
C
V
V
I
L = 1.1µH
LOAD
LOAD
OUT
OUT
IN
OUT
OUT
IN
= 7V
= 14V
= 47µF
= 0.78V
= 47µF
= 0.78V
= 1.1A
= 1.1A
(up to 20V), but the output voltage ripple
IN
and BOOST pins when the output is in
Figure 1
Figure 2
1µs/DIV
1µs/DIV
IN
3503 F01
3503 F02
= 14V.
IN(PS)
3503f