LTC1574-3.3 LINER [Linear Technology], LTC1574-3.3 Datasheet - Page 4

• Manufacturer Part Number: LTC1574-3.3
• Description: High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode
• Manufacturer: LINER [Linear Technology]

PIN
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
close to ground (Pin 4).
I
P-channel switch. With I
600mA and with I
reduced to 340mA.
SHDN (Pin 7): Pulling this pin to ground keeps the internal
switch off and puts the LTC1574 in micropower shutdown.
APPLICATIONS
LTC1574
LTC1574-3.3/LTC1574-5
Operating Frequency and Inductor
Since the LTC1574 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
frequency of operation can be expressed as:
where t
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:
By choosing a smaller inductor, a low ESR (Effective Series
Resistance) output filter capacitor has to be used. Core loss
will increase due to higher ripple current.
Short-Circuit Protection
The LTC1574 is protected from output short circuits by its
internal current limit. Depending on the condition of the
4
PGM
IN
I
U
f
RIPPLE
(Pin 5): Input Supply Voltage. It must be decoupled
(Pin 6): This pin selects the current limit of the
FUNCTIONS
t
OFF
OFF
1
U
= 4 s and V
V
4 10
IN
V
IN
V
PGM
U
OUT
V
D
U
6
PGM
V
D
= 0V, the current trip point is
INFORMATION
OUT
U
is the voltage drop across the
= V
Hz
L
IN
V
, the current trip point is
D
W
A
P-P
U
IN
. The
V
to 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
This pin will sink current when (Pin 12) LB
1.25V.
LB
Comparator. The (+) input is connected to a reference
voltage of 1.25V.
I
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).
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
PGM
OUT
OUT
IN
I
PGM
I
PGM
(Pin 12): The (–) Input of the Low-Battery Voltage
or V
pin, the limit is either set to 340mA or 600mA. In
= V
GND
(Pin 11): Open drain of an N-Channel Pull-Down.
= 0
IN
Figure 1. Inductor Current with Output Shorted
FB
L = 100 H
V
IN
(Pin 10): For the LTC1574, this pin connects
= 13.5V
20 s/DIV
IN
goes below
1574 • F01