Datasheet

4
LTC1574
LTC1574-3.3/LTC1574-5
V
OUT
or V
FB
(Pin 10): For the LTC1574, this pin connects
t o the main voltage comparator input. On the LTC1574-5
and LTC1574-3.3, this pin goes to an internal resistive
divider which sets the output voltage .
LB
OUT
(Pin 11): Open drain of an N-Channel Pull-Down.
This pin will sink current when (Pin 12) LB
IN
goes below
1.25V.
LB
IN
(Pin 12): The (–) Input of the Low-Battery Voltage
Comparator. The (+) input is connected to a reference
voltage of 1.25V.
NC (Pins 1, 8, 9, 16): No Connection.
GND (Pins 2, 4, 13, 15): Ground.
SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and
Cathode of Schottky Diode.
V
IN
(Pin 5): Input Supply Voltage. It must be decoupled
close to ground (Pin 4).
I
PGM
(Pin 6): This pin selects the current limit of the
P-channel switch. With I
PGM
= V
IN
, the current trip point is
600mA and with I
PGM
= 0V, the current trip point is
reduced to 340mA.
SHDN (Pin 7): Pulling this pin to ground keeps the internal
switch off and puts the LTC1574 in micropower shutdown.
Operating Frequency and Inductor
Since the LTC1574 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
IN
. The
frequency of operation can be expressed as:
f
t
VV
VV
Hz
OFF
IN OUT
IN D
=
−
+






()
1
where t
OFF
= 4µs and V
D
is the voltage drop across the
internal Schottky diode. Note that the operating frequency
is a function of the input and output voltage.
Although the size of the inductor does not affect the fre-
quency or inductor peak current, it does affect the ripple
current. The peak-to-peak ripple current is given by:
I
VV
L
RIPPLE
OUT D
=
+






()
−
410
6
• A
P-P
When choosing a small inductor, core loss will increase due
to higher ripple current. Therefore, a low ESR output
capacitor has to be used.
Short-Circuit Protection
The LTC1574 is protected from output short circuits by its
internal current limit. Depending on the condition of the
Low-Battery Detector
The low-battery indicator senses the input voltage through
an external resistive divider. This divided voltage connects
to the “–” input of a voltage comparator (Pin 12) which is
compared with a 1.25V reference voltage. With the current
I
PGM
pin, the limit is either set to 340mA or 600mA. In
addition, the off-time of the switch is increased to allow the
inductor current to decay far enough to prevent any current
build-up (see Figure 1).
I
PGM
= V
IN
I
PGM
= 0
GND
L = 100µH
V
IN
= 13.5V
100mA/DIV
20µs/DIV
1574 • F01
Figure 1. Inductor Current with Output Shorted
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PI FU CTIO S
APPLICATIO S I FOR ATIO
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