LT1777
Low Noise Step-Down
Switching Regulator
FEATURES
s
s
s
s
s
s
s
s
s
DESCRIPTIO
Programmable dI/dt Limit
Internally Limited dV/dt
High Input Voltage: 48V Max
700mA Peak Switch Rating
True Current Mode Control
100kHz Fixed Operating Frequency
Synchronizable to 250kHz
Low Supply Current in Shutdown: 30µA
Low Thermal Resistance 16-Pin SO Package
The LT
®
1777 is a Buck (step-down) regulator designed for
noise sensitive applications. It contains a dI/dt limiting
circuit programmed via a small external inductor in the
switching path. Internal circuitry also generates controlled
dV/dt ramp rates.
The monolithic die includes all oscillator, control and
protection circuitry. The part can accept operating input
voltages as high as 48V, and contains an output switch
rated at 700mA peak current. Current mode control offers
excellent dynamic input supply rejection and short-circuit
protection. The internal control circuitry is normally pow-
ered via the V
CC
pin, thereby minimizing power drawn
directly from the V
IN
supply (see Applications Informa-
tion). The fused-lead SO16 package and 100kHz switch-
ing frequency allow for minimal PC board area
requirements.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
Automotive Cellular and GPS Receivers
Telecom Power Supplies
Industrial Instrument Power Supplies
TYPICAL APPLICATIO
Low Noise 5V Step-Down Supply
V
IN
24V
+
10
39µF
63V
100pF
12
14
100pF 12k
SGND
2200pF
7
12.1k
1%
1777 TA01
3
V
IN
SHDN
V
CC
V
SW
V
D
FB
LT1777
SYNC
V
C
4
1µH*
6
5
13
MBRS1100
220µH
V
OUT
5V
400mA
100µF
10V
36.5k
1%
V
SW
VOLTAGE
10V/DIV
+
V
SW
CURRENT
200mA/DIV
1777 TA02
*PROGRAMS dI/dT
U
V
SW
Switching Waveforms
500ns/DIV
U
U
1
LT1777
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GND* 1
NC 2
SHDN 3
V
CC
4
V
D
5
V
SW
6
SGND 7
GND* 8
16 GND*
15 NC
14 V
C
13 FB
12 SYNC
11 NC
10 V
IN
9
GND*
Supply Voltage ....................................................... 48V
Switch Voltage (V
IN
– V
SW
) (Note 4) ...................... 51V
SHDN, SYNC Pin Voltage .......................................... 7V
V
CC
Pin Voltage ...................................................... 30V
FB Pin Voltage ........................................................ 3.0V
Operating Junction Temperature Range
LT1777C ............................................... 0°C to 125°C
LT1777I ........................................... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LT1777CS
LT1777IS
S PACKAGE
16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 50°C/W*
*FOUR CORNER PINS ARE
FUSED TO INTERNAL DIE
ATTACH PADDLE FOR HEAT
SINKING. CONNECT THESE
FOUR PINS TO EXPANDED PC
LANDS FOR PROPER HEAT
SINKING.
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
J
= 25°C.
V
IN
= 24V, V
SW
Open, V
CC
= 5V, V
C
= 1.4V unless otherwise noted.
SYMBOL
V
IN(MIN)
I
VIN
I
VCC
V
VCC
PARAMETER
Minimum Input Voltage
q
CONDITIONS
MIN
TYP
6.7
MAX
7.0
7.4
800
900
3.5
4.5
3.1
50
75
1.255
1.265
1500
1000
1500
170
220
0.01
UNITS
V
V
µA
µA
mA
mA
V
µA
µA
V
V
nA
µmho
µmho
µA
µA
V
%/V
V/V
Power Supplies
V
IN
Supply Current
V
CC
Supply Current
V
CC
Dropout Voltage
Shutdown Mode I
VIN
V
C
= 0V
q
620
2.5
q
q
q
V
C
= 0V
(Note 2)
V
SHDN
= 0V
2.8
30
Feedback Amplifier
V
REF
I
IN
g
m
I
SRC
, I
SNK
V
CL
Reference Voltage
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
∆I
C
=
±10µA
q
650
100
2.0
Feedback Amplifier Clamp Voltage
Reference Voltage Line Regulation
Voltage Gain
12V
≤
V
IN
≤
48V
q
200
600
2
U
W
U
U
W W
W
LT1777
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
J
= 25°C.
V
IN
= 24V, V
SW
Open, V
CC
= 5V, V
C
= 1.4V unless otherwise noted.
SYMBOL
R
ON
I
LIM
PARAMETER
Output Switch On Voltage
Switch Current Limit
Output dl/dt Sense Voltage
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
I
SW
= 0.5A
(Note 3)
q
MIN
TYP
1.0
MAX
1.5
1.0
2.0
UNITS
V
A
V
V
V
A/V
Output Switch
0.55
0.6
0.9
1.1
2
90
85
85
100
90
1.5
130
40
0.5
q
0.70
1.3
Current Amplifier
Control Pin Threshold
Control Voltage to Switch Transconductance
Timing
f
Switching Frequency
q
Duty Cycle = 0%
1.25
110
115
kHz
kHz
%
Maximum Switch Duty Cycle
Sync Function
Minimum Sync Amplitude
Synchronization Range
SYNC Pin Input R
SHDN Pin Function
V
SHDN
Shutdown Mode Threshold
q
q
q
2.2
250
V
kHz
kΩ
V
V
V
V
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 dI/dt rate will cause a somewhat higher current limit in actual
application.
Note 4:
During normal operation the V
SW
pin may fly as much as 3V
below ground. However, the LT1777 may not be used in an inverting
DC/DC configuration.
3
LT1777
TYPICAL PERFOR A CE 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)
50
25
75
0
TEMPERATURE (°C)
Switch Current Limit
vs Duty Cycle
1000
T
A
= 25°C
900
800
SWITCH CURRENT LIMIT (mA)
SHDN PIN VOLTAGE (mV)
800
600
400
200
300
0
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1777 G03
SHDN Pin Current vs Voltage
5
1.30
SHDN PIN INPUT CURRENT (µA)
0
SHDN PIN VOLTAGE (V)
–5
–10
–15
–20
0
1
3
4
2
SHDN PIN VOLTAGE (V)
5
1777 G05
4
U W
Switch On Voltage
vs Switch Current
1.50
1.25
25°C
1.00
0.75
0.50
0.25
0
– 55°C
125°C
100
125
0
100
200 300 400 500 600
SWITCH CURRENT (mA)
700
1777
G01
1777 G02
SHDN Pin Shutdown Threshold
vs Temperature
700
600
500
400
200
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1777
G04
SHDN Pin Lockout Thresholds
vs Temperature
1.28
UPPER THRESHOLD
1.26
LOWER THRESHOLD
1.24
25°C
125°C
–55°C
1.22
1.20
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1777 G06
LT1777
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V)
106
104
102
100
98
96
94
–50 –25
SWITCHING FREQUENCY (kHz)
0
25
TEMPERATURE (°C)
1777 G07
Output dI/dt Sense Voltage
vs Temperature
2.0
1.8
2.2
2.0
V
C
PIN VOLTAGE (V)
dI/dt SENSE VOLTAGE (V)
1.6
1.4
1.2
1.0
0.8
0.6
–50 –25
50
25
75
0
TEMPERATURE (°C)
Feedback Amplifier Output
Current vs FB Pin Voltage
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
100
25°C
125°C
–55°C
750
700
650
600
550
500
450
–150
1.0
50
0
–50
–100
TRANSCONDUCTANCE (µmho)
1.1
1.3
1.4
1.2
FB PIN VOLTAGE (V)
U W
50
75
Minimum Synchronization
Voltage vs Temperature
2.25
2.00
1.75
1.50
1.25
1.00
0.75
–50 –25
100
125
0
25
50
75
100
125
TEMPERATURE (°C)
1777 G08
V
C
Pin Switching Threshold,
Clamp Voltage vs Temperature
1.8
1.6
1.4
1.2
1.0
0.8
–50 –25
SWITCHING
THRESHOLD
CLAMP
VOLTAGE
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
1777 G09
1777 G10
Error Amplifier Transconductance
vs Temperature
1.5
1777 G11
400
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1777
G12
5
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
