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LT1371CSW#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
4
LT1371
TYPICAL PERFORMANCE CHARACTERISTICS
U
W
Error Amplifier Output Current
vs Feedback Pin Voltage
Shutdown Delay and Threshold
vs Temperature
TEMPERATURE (°C)
–50
0
SHUTDOWN DELAY (µs)
SHUTDOWN THRESHOLD (V)
2
6
8
10
20
14
0
50
75
LT1371 • G04
4
16
18
12
0
0.2
0.6
0.8
1.0
2.0
1.4
0.4
1.6
1.8
1.2
–25 25
100
125
150
SHUTDOWN THRESHOLD
SHUTDOWN DELAY
S/S or SHDN Pin Input Current
vs Voltage
Error Amplifier Transconductance
vs Temperature
Switching Frequency
vs Feedback Pin Voltage
FEEDBACK PIN VOLTAGE (V)
400
ERROR AMPLIFIER OUTPUT CURRENT (µA)
–300
–200
–100
300
100
–0.1 0.1
200
0
–0.3 –0.2
V
REF
–55°C
125°C
25°C
LT1371 • G06
Minimum Synchronization
Voltage vs Temperature
TEMPERATURE (°C)
–50
0
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
0.5
1.0
1.5
2.0
050
100
150
LT1371 • G05
2.5
3.0
–25 25
75
125
f
SYNC
= 700kHz
VOLTAGE (V)
–1
INPUT CURRENT (µA)
1
3
5
7
LT1371 • G07
–1
–3
0
2
4
–2
–4
–5
1
3
5
08
2
4
6
9
V
IN
= 5V
FEEDBACK PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (% OF TYPICAL)
70
90
110
0.8
LT1371 • G08
50
30
60
80
100
40
20
10
0.2
0.4
0.6
0.1 0.9
0.3
0.5
0.7
1.0
TEMPERATURE (°C)
–50
0
TRANSCONDUCTANCE (µmho)
200
600
800
1000
2000
1400
0
50
75
LT1371 • G09
400
1600
1800
1200
–25 25
100
125
150
g
m
=
∆I (V
C
)
∆V (FB)
Switch Saturation Voltage
vs Switch Current
TEMPERATURE (°C)
–50
1.8
INPUT VOLTAGE (V)
2.0
2.2
2.4
2.6
050
100
150
LT1371 • G03
2.8
3.0
–25 25
75
125
Minimum Input Voltage
vs Temperature
DUTY CYCLE (%)
0
SWITCH CURRENT LIMIT (A)
2
4
6
1
3
5
20 40 60 80
LT1371 • G02
10010
0
30 50 70 90
25°C AND
125°C
–55°C
Switch Current Limit
vs Duty Cycle
SWITCH CURRENT (A)
0
SWITCH SATURATION VOLTAGE (V)
0.6
0.8
1.0
3.2
LT1371 • G01
0.4
0.2
0.5
0.7
0.9
0.3
0.1
0
0.8
1.6
2.4
4.0
2.8
0.4
1.2
2.0
3.6
100°C
150°C
25°C
–55°C
5
LT1371
TYPICAL PERFORMANCE CHARACTERISTICS
U
W
TEMPERATURE (°C)
–50
FEEDBACK INPUT CURRENT (nA)
400
500
600
150
LT1371 • G11
300
200
0
0
50
100
100
800
700
–25
25
75
125
V
FB
=V
REF
Feedback Input Current
vs Temperature
TEMPERATURE (°C)
–50
–50
NEGATIVE FEEDBACK INPUT CURRENT (µA)
–30
0
0
50
75
LT1371 • G12
–40
–10
–20
–25 25
100
125
150
V
NFB
=V
NFR
Negative Feedback Input Current
vs Temperature
V
C
Pin Threshold and High
Clamp Voltage vs Temperature
TEMPERATURE (°C)
–50
0.4
V
C
PIN VOLTAGE (V)
0.6
1.0
1.2
1.4
2.4
1.8
0
50
75
LT1371 • G10
0.8
2.0
2.2
1.6
–25 25
100
125
150
V
C
HIGH CLAMP
V
C
THRESHOLD
PIN FUNCTIONS
UUU
V
C
: The Compensation pin is used for frequency compen-
sation, current limiting and soft start. It is the output of the
error amplifier and the input of the current comparator.
Loop frequency compensation can be performed with an
RC network connected from the V
C
pin to ground.
FB: T
he Feedback pin is used for positive output voltage
sensing and oscillator frequency shifting. It is the invert-
ing input to the error amplifier. The noninverting input of
this amplifier is internally tied to a 1.245V reference. Load
on the FB pin should not exceed 250µA when NFB pin is
used. See Applications Information.
NFB: The Negative Feedback pin is used for negative
output voltage sensing. It is connected to the inverting
input of the negative feedback amplifier through a 100k
source resistor.
S/S (R and T7 Packages Only): Shutdown and Synchroni-
zation Pin. The S/S pin is logic level compatible. Shutdown
is active low and the shutdown threshold is typically 1.3V.
For normal operation, pull the S/S pin high, tie it to V
IN
or
leave it floating. To synchronize switching, drive the S/S
pin between 600kHz and 800kHz.
SHDN: (SW Package Only): The Shutdown pin is active
low and the shutdown threshold is typically 1.3V. For
normal operation, pull the SHDN pin high, tie it to V
IN
or
leave it floating.
SYNC (SW Package Only): To synchronize switching,
drive the SYNC pin between 600kHz and 800kHz. If not
used, the SYNC pin can be tied high, low or left floating.
V
IN
: Bypass Input Supply pin with a low ESR capacitor,
10µF or more. The regulator goes into undervoltage lock-
out when V
IN
drops below 2.5V. Undervoltage lockout
stops switching and pulls the V
C
pin low.
V
SW
: The Switch pin is the collector of the power switch
and has large currents flowing through it. Keep the traces
to the switching components as short as possible to
minimize radiation and voltage spikes.
GND: Tie all Ground pins to a good quality ground plane.
6
LT1371
BLOCK DIAGRAM
W
OPERATION
U
The LT1371 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage. Referring to the block
diagram, the switch is turned ON at the start of each
oscillator cycle. It is turned OFF when switch current
reaches a predetermined level. Control of output voltage is
obtained by using the output of a voltage sensing error
amplifier to set current trip level. This technique has
several advantages. First, it has immediate response to
input voltage variations, unlike voltage mode switchers
which have notoriously poor line transient response.
Second, it reduces the 90° phase shift at mid-frequencies
in the energy storage inductor. This greatly simplifies
closed-loop frequency compensation under widely vary-
ing input voltage or output load conditions. Finally, it
allows simple pulse-by-pulse current limiting to provide
maximum switch protection under output overload or
short conditions. A low dropout internal regulator pro-
vides a 2.3V supply for all internal circuitry. This low
dropout design allows input voltage to vary from 2.7V to
25V with virtually no change in device performance. A
500kHz oscillator is the basic clock for all internal timing.
It turns ON the output switch via the logic and driver
circuitry. Special adaptive anti-sat circuitry detects onset
of saturation in the power switch and adjusts driver
current instantaneously to limit switch saturation. This
minimizes driver dissipation and provides very rapid turn-
off of the switch.
A 1.245V bandgap reference biases the positive input of
the error amplifier. The negative input of the amplifier is
brought out for positive output voltage sensing. The error
amplifier has nonlinear transconductance to reduce out-
put overshoot on start-up or overload recovery. When
the feedback voltage exceeds the reference by 40mV,
error amplifier transconductance increases 10 times,
which reduces output overshoot. The feedback input also
invokes oscillator frequency shifting, which helps pro-
tect components during overload conditions. When the
feedback voltage drops below 0.6V, the oscillator fre-
quency is reduced 5:1. Lower switching frequency allows
full control of switch current limit by reducing minimum
switch duty cycle.
–
+
NFBA
NFB
SHDN
S/S*
SYNC
FB
100k
50k
0.04Ω
–
+
EA
V
C
V
IN
GND
LT1371 • BD
GND SENSE *R AND T7 PACKAGES ONLY
1.245V
REF
5:1 FREQUENCY
SHIFT
OSCSYNC
SHUTDOWN
DELAY AND RESET
LOW DROPOUT
2.3V REG
ANTI-SAT
LOGIC DRIVER
SW
SWITCH
–
+
IA
A
V
≈ 6
COMP
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT1371CSW#PBF

Mfr. #:
Buy LT1371CSW#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 500kHz Hi Eff 3A Sw Reg
Lifecycle:
New from this manufacturer.
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