LT1578 Linear Technology, LT1578 Datasheet - Page 8

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1578/LT1578-2.5
APPLICATIONS
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1578 is used to set output
voltage and provide several overload protection features.
The first part of this section deals with selecting resistors
to set output voltage and the remaining part talks about
foldback frequency and current limiting created by the FB
pin. Please read both parts before committing to a final
design. The fixed 2.5V LT1578-2.5 has internal divider
resistors and the FB pin, renamed SENSE, is connected
directly to the 2.5V output.
The suggested value for the output divider resistor (see
Figure 2) from FB to ground (R2) is 5k or less, and a
formula for R1 is shown below. The output voltage error
caused by ignoring the input bias current on the FB pin is
less than 0.25% with R2 = 5k. Please read the following
if divider resistors are increased above the suggested
values.
8
R
1
R V
2
OUT
1 21
U
1 21
LT1578
INFORMATION
V
U
C
Q2
GND
AMPLIFIER
W
R5
ERROR
5k
TO SYNC CIRCUIT
TO FREQUENCY
Figure 2. Frequency and Current Limit Foldback
SHIFTING
+
–
1.4V
U
1.21V
Q1
R3
1k
More Than Just Voltage Feedback
The feedback pin is used for more than just output voltage
sensing. It also reduces switching frequency and current
limit when output voltage is very low (see the Frequency
Foldback graph in Typical Performance Characteristics).
This is done to control power dissipation in both the IC and
the external diode and inductor during short-circuit con-
ditions. A shorted output requires the switching regulator
to operate at very low duty cycles, and the average current
through the diode and inductor is equal to the short-circuit
current limit of the switch (typically 2A for the LT1578,
folding back to less than 0.77A). Minimum switch on time
limitations would prevent the switcher from attaining a
sufficiently low duty cycle if switching frequency were
maintained at 200kHz, so frequency is reduced by about
5:1 when the feedback pin voltage drops below 0.7V (see
Frequency Foldback graph). This does not affect operation
with normal load conditions; one simply sees a gear shift
in switching frequency during start-up as the output
voltage rises.
R4
1k
V
SW
FB
R1
R2
5k
+
OUTPUT
5V
1578 F02