LT1371 Linear Technology, LT1371 Datasheet - Page 7

• Manufacturer Part Number: LT1371
• Description: 500kHz High Efficiency 3A Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
APPLICATIO S I FOR ATIO
Unique error amplifier circuitry allows the LT1371 to
directly regulate negative output voltages. The negative
feedback amplifier’s 100k source resistor is brought out
for negative output voltage sensing. The NFB pin regulates
at – 2.49V while the amplifier output internally drives the
FB pin to 1.245V. This architecture, which uses the same
main error amplifier, prevents duplicating functions and
maintains ease of use. Consult LTC Marketing for units
that can regulate down to – 1.25V.
The error signal developed at the amplifier output is
brought out externally. This pin (V
Positive Output Voltage Setting
The LT1371 develops a 1.245V reference (V
FB pin to ground. Output voltage is set by connecting the
FB pin to an output resistor divider (Figure 1). The FB pin
bias current represents a small error and can usually be
ignored for values of R2 up to 7k. The suggested value for
R2 is 6.19k. The NFB pin is normally left open for positive
output applications. Positive fixed voltage versions are
available (consult LTC Marketing).
Negative Output Voltage Setting
The LT1371 develops a – 2.49V reference (V
NFB pin to ground. Output voltage is set by connecting the
NFB pin to an output resistor divider (Figure 2). The
–30 A NFB pin bias current (I
voltage errors and should not be ignored. This has been
accounted for in the formula in Figure 2. The suggested
value for R2 is 2.49k. The FB pin is normally left open for
negative output applications. See Dual Polarity Output
Voltage Sensing for limitations on FB pin loading when
using the NFB pin.
Dual Polarity Output Voltage Sensing
Certain applications benefit from sensing both positive
and negative output voltages. One example is the “Dual
Output Flyback Converter with Overvoltage Protection”
circuit shown in the Typical Applications section. Each
output voltage resistor divider is individually set as de-
scribed above. When both the FB and NFB pins are used,
U U
U U
NFB
C
W
W
) has three different
) can cause output
NFR
REF
) from the
) from the
U
U
functions. It is used for frequency compensation, current
limit adjustment and soft starting. During normal regula-
tor operation this pin sits at a voltage between 1V (low
output current) and 1.9V (high output current). The error
amplifier is a current output (g
be externally clamped for lowering current limit. Like-
wise, a capacitor coupled external clamp will provide soft
start. Switch duty cycle goes to zero if the V
below the control pin threshold, placing the LT1371 in an
idle mode.
the LT1371 acts to prevent either output from going
beyond its set output voltage. For example, in this applica-
tion if the positive output were more heavily loaded than
the negative, the negative output would be greater and
would regulate at the desired set-point voltage. The posi-
tive output would sag slightly below its set-point voltage.
This technique prevents either output from going unregu-
lated high at no load. Please note that the load on the FB
pin should not exceed 250 A when the NFB pin is used.
This situation occurs when the resistor dividers are used
at both FB and NFB. True load on FB is not the full divider
current unless the positive output is shorted to ground.
See Dual Output Flyback Converter application.
NFB
PIN
Figure 2. Negative Output Resistor Divider
Figure 1. Positive Output Resistor Divider
V
NFR
PIN
FB
I
V
NFB
REF
–V
OUT
R1
R2
V
OUT
–V
R1 =
R1
R2
OUT
2.49
m
V
R1 = R2
= V
R2
OUT
) type, so this voltage can
NFB
V
= V
OUT
+ 30 • 10
1 +
REF
1.245
V
– 2.49
OUT
R1
R2
LT1371 • F01
1 +
– 6
– 1
+ I
R1
R2
NFB
C
pin is pulled
LT1371
(R1)
LT1371 • F02
7