LT1776
Wide Input Range,
High Efficiency, Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1776 is a wide input range, high efficiency Buck
(step-down) switching regulator. The monolithic die in-
cludes all oscillator, control and protection circuitry. The
part can accept input voltages as high as 60V and contains
an output switch rated at 700mA peak current. Current
mode control delivers excellent dynamic input supply
rejection and short-circuit protection.
The LT1776 contains several features to enhance effi-
ciency. The internal control circuitry is normally powered
via the V
CC
pin, thereby minimizing power drawn directly
from the V
IN
supply (see Applications Information). The
action of the LT1776 switch circuitry is also load depen-
dent. At medium to high loads, the output switch circuitry
maintains fast rise time for good efficiency. At light loads,
rise time is deliberately reduced to avoid pulse skipping
behavior.
The available SO-8 package and 200kHz switching fre-
quency allow for minimal PC board area requirements.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Wide Input Range: 7.4V to 40V
Tolerates Input Transients to 60V
700mA Peak Switch Rating
Adaptive Switch Drive Maintains Efficiency at High
Load Without Pulse Skipping at Light Load
True Current Mode Control
200kHz Fixed Operating Frequency
Synchronizable to 400kHz
Low Supply Current in Shutdown: 30µA
Available in 8-Pin SO and PDIP Packages
APPLICATIONS
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Automotive DC/DC Converters
Cellular Phone Battery Charger Accessories
IEEE 1394 Step-Down Converters
TYPICAL APPLICATION
V
IN
8V TO 40V
1
5
V
IN
SHDN
V
CC
V
SW
39µF
63V
6
LT1776
FB
8
V
C
2200pF
22k
100pF
12.1k
1%
7
2
3
100µH*
5V
400mA
36.5k
1%
90
80
MBR160
100µF
10V
EFFICIENCY (%)
+
+
70
60
50
40
SYNC
GND
4
*43T #30 ON MAGNETICS
MPP #55030
1776 F01
V
IN
= 10V
V
IN
= 20V
30
V
IN
= 30V
V
IN
= 40V
20
1
Figure 1
U
U
U
Efficiency vs V
IN
and I
LOAD
10
100
LOAD CURRENT (mA)
1000
1776 TA01
1
LT1776
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SHDN 1
V
CC
2
V
SW
3
GND 4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
V
C
FB
SYNC
V
IN
Supply Voltage (Note 5) .......................................... 60V
Switch Voltage (Note 5) ........................................... 60V
SHDN, SYNC Pin Voltage ........................................... 7V
V
CC
Pin Voltage ....................................................... 30V
FB Pin Voltage ........................................................... 3V
Operating Junction Temperature Range
LT1776C ................................................ 0°C to 125°C
LT1776I ............................................ – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1776CN8
LT1776CS8
LT1776IN8
LT1776IS8
S8 PART MARKING
1776
1776I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/ W (N8)
T
JMAX
= 125°C,
θ
JA
= 110°C/ W (S8)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
IN
= 40V, V
SW
open, V
CC
= 5V, V
C
= 1.4V unless otherwise noted.
SYMBOL
V
IN(MIN)
PARAMETER
Minimum Input Voltage
q
CONDITIONS
MIN
TYP
6.7
MAX
7.0
7.4
800
900
4.0
5.0
3.1
50
75
1.255
1.265
1500
1000
1500
170
220
0.01
UNITS
V
V
V
µA
µA
mA
mA
V
µA
µA
V
V
nA
µmho
µmho
µA
µA
V
%/V
V/V
Power Supplies
Thermally Limited Continuous Operating Voltage
I
VIN
I
VCC
V
VCC
V
IN
Supply Current
V
CC
Supply Current
V
CC
Dropout Voltage
Shutdown Mode I
VIN
Feedback Amplifier
V
REF
I
IN
g
m
I
SRC
, I
SNK
V
CL
Reference Voltage
V
C
= 0V
q
q
40
620
3.2
V
C
= 0V
q
(Note 2)
V
SHDN
= 0V
q
2.8
30
q
1.225
1.215
400
200
60
45
1.240
600
FB Pin Input Bias Current
Feedback Amplifier Transconductance
Feedback Amplifier Source or Sink Current
q
∆lc
=
±10µA
q
650
100
2.0
Feedback Amplifier Clamp Voltage
Reference Voltage Line Regulation
Voltage Gain
12V
≤
V
IN
≤
60V
q
200
I
SW
= 0.5A
(Note 3)
Duty Cycle = 0%
q
600
1.0
1.5
1.0
1.25
Output Switch
V
ON
I
LIM
Output Switch On Voltage
Switch Current Limit
Control Pin Threshold
Control Voltage to Switch Transconductance
V
A
V
A/V
0.55
0.9
0.70
1.1
2
Current Amplifier
2
U
W
U
U
W W
W
LT1776
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
IN
= 40V, V
SW
open, V
CC
= 5V, V
C
= 1.4V unless otherwise noted.
SYMBOL
Timing
f
Switching Frequency
q
PARAMETER
CONDITIONS
MIN
180
170
85
TYP
200
90
300
1.35
0.2
1.6
MAX
220
230
UNITS
kHz
kHz
%
ns
V
V/ns
V/ns
Maximum Switch Duty Cycle
t
ON(MIN)
Minimum Switch On Time
V
C
Pin Boost Threshold
dV/dt Below Threshold
dV/dt Above Threshold
Sync Function
Minimum Sync Amplitude
Synchronization Range
SYNC Pin Input R
SHDN Pin Function
V
SHDN
Shutdown Mode Threshold
(Note 6)
High dV/dt Mode, R
L
= 39Ω (Note 4)
Boost Operation
q
q
q
1.5
250
40
0.5
2.2
400
V
kHz
kΩ
V
V
V
V
q
0.2
1.260
1.245
12
2.5
0.8
Upper Lockout Threshold
Lower Lockout Threshold
I
SHDN
Shutdown Pin Current
Switching Action On
Switching Action Off
V
SHDN
= 0V
V
SHDN
= 1.25V
20
10
µA
µA
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Control circuitry powered from V
CC
.
Note 3:
Switch current limit is DC trimmed and tested in production.
Inductor dl/dt rate will cause a somewhat higher current limit in actual
application.
Note 4:
Minimum switch on time is production tested with a 39Ω resistive
load to ground.
Note 5:
Parts are guaranteed to survive 60V on V
IN
and V
SW
. However,
thermal constraints will limit V
IN
in some applications, depending primarily
on maximum output current and switching frequency. See Applications
section for more information.
Note 6:
Internal oscillator is guaranteed to sync up to 400kHz. However,
thermal constraints and/or controllability issues may place a lower limit on
switching frequency in actual usage. See Applications section for more
information.
3
LT1776
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage vs
Temperature
7.4
7.2
INPUT VOLTAGE (V)
7.0
6.8
6.6
6.4
6.2
6.0
–50 –25
SWITCH VOLTAGE (V)
1.50
1.25
25°C
1.00
0.75
0.50
0.25
0
– 55°C
SWITCH CURRENT LIMIT (mA)
50
25
75
0
TEMPERATURE (°C)
SHDN Pin Shutdown Threshold
vs Temperature
900
SHDN PIN INPUT CURRENT (µA)
5
800
SHDN PIN VOLTAGE (mV)
700
600
500
400
300
200
–50 –25
SHDN PIN VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
Switching Frequency
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V)
215
210
205
200
195
190
185
–50 –25
2.25
2.00
1.75
1.50
1.25
1.00
SWITCH MINIMUM ON-TIME (ns)
SWITCHING FREQUENCY (kHz)
0
25
50
75
TEMPERATURE (°C)
1776 G07
4
U W
100
1776 G01
Switch-On Voltage vs
Switch Current
1000
Switch Current Limit vs
Duty Cycle
T
A
= 25°C
800
600
125°C
400
200
0
0
100
200 300 400 500 600
SWITCH CURRENT (mA)
700
125
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1776 G03
1776 G02
SHDN Pin Input Current
vs Voltage
1.30
SHDN Pin Lockout Thresholds
vs Temperature
0
1.28
UPPER THRESHOLD
1.26
LOWER THRESHOLD
1.24
–5
–10
–15
25°C
–55°C
125°C
0
1
3
4
2
SHDN PIN VOLTAGE (V)
5
1776 G05
1.22
–20
100
125
1.20
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
LT1776
G04
LT1776
G06
Minimum Synchronization Voltage
vs Temperature
600
500
400
300
200
100
Switch Minimum On-Time
vs Temperature
V
IN
= 40V
R
L
= 39Ω
FB =
100
125
0.75
–50 –25
0
25
50
75
100
125
0
–50 –25
TEMPERATURE (°C)
1776 G08
50
25
75
0
TEMPERATURE (°C)
100
125
1776 G09
LT1776
TYPICAL PERFORMANCE CHARACTERISTICS
V
C
Pin Switching Threshold,
Boost Threshold, Clamp Voltage
vs Temperature
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
2.2
2.0
V
C
PIN VOLTAGE (V)
1.8
1.6
1.4
1.2
1.0
0.8
–50 –25
BOOST
THRESHOLD
SWITCHING
THRESHOLD
CLAMP
VOLTAGE
50
TRANSCONDUCTANCE (µmho)
50
25
75
0
TEMPERATURE (°C)
PIN FUNCTIONS
SHDN (Pin 1):
When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regulator
and reduces V
IN
input current to a few tens of microam-
peres (shutdown mode).
When this pin is held above the shutdown mode threshold,
but below the lockout threshold, the part will be opera-
tional with the exception that output switching action will
be inhibited (lockout mode). A user-adjustable undervolt-
age lockout can be implemented by driving this pin from
an external resistor divider to V
IN
. This action is logically
“ANDed” with the internal UVLO, set at nominally 6.7V,
such that minimum V
IN
can be increased above 6.7V, but
not decreased (see Applications Information).
If unused, this pin should be left open. However, the high
impedance nature of this pin renders it susceptible to
coupling from the high speed V
SW
node, so a small
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open”.
V
CC
(Pin 2):
This pin is used to power the internal control
circuitry off of the switching supply output. Proper use of
this pin enhances overall power supply efficiency. During
start-up conditions, internal control circuitry is powered
directly from V
IN
.
If the output capacitor is located more than one inch from
the V
CC
pin, a separate 0.1µF bypass capacitor to ground
may be required right at the pin.
V
SW
(Pin 3):
This is the emitter node of the output switch
and has large currents flowing through it. This node
moves at a high dV/dt rate, especially when in “boost”
mode. Keep the traces to the switching components as
short as possible to minimize electromagnetic radiation
and voltage spikes.
GND (Pin 4):
This is the device ground pin. The internal
reference and feedback amplifier are referred to it. Keep
the ground path connection to the FB divider and the V
C
compensation capacitor free of large ground currents.
V
IN
(Pin 5):
This is the high voltage supply pin for the
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the V
CC
pin is left
open. A high quality bypass capacitor which meets the
input ripple current requirements is needed here. (See
Applications Information).
SYNC (Pin 6):
Pin used to synchronize internal oscillator
to the external frequency reference. It is directly logic
compatible and can be driven with any signal between
U W
100
125
LT1776
G10
Feedback Amplifier Output
Current vs FB Pin Voltage
100
25°C
–55°C
125°C
750
700
650
600
550
500
450
Error Amplifier Transconductance
vs Temperature
0
–50
–100
–150
1.0
1.1
1.3
1.4
1.2
FB PIN VOLTAGE (V)
1.5
1776 G11
400
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
LT1776
G12
U
U
U
5
嵌入式系统
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