LT1676
Wide Input Range,
High Efficiency, Step-Down
Switching Regulator
FEATURES
s
s
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DESCRIPTIO
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s
Wide Input Range: 7.4V to 60V
700mA Peak Switch Current Rating
Adaptive Switch Drive Maintains Efficiency at High
Load Without Pulse Skipping at Light Load
True Current Mode Control
100kHz Fixed Operating Frequency
Synchronizable to 250kHz
Low Supply Current in Shutdown: 30µA
Available in 8-Pin SO and PDIP Packages
The LT
®
1676 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 offers excellent dynamic input supply rejec-
tion and short-circuit protection.
The LT1676 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 LT1676 switch circuitry is also load depen-
dent. At medium to high loads, the output switch circuitry
maintains high rise time for good efficiency. At light loads,
rise time is deliberately reduced to avoid pulse skipping
behavior.
The available SO-8 package and 100kHz switching fre-
quency allow for minimal PC board area requirements.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Automotive DC/DC Converters
Telecom 48V Step-Down Converters
Cellular Phone Battery Charger Accessories
IEEE 1394 Step-Down Converters
TYPICAL APPLICATIO
V
IN
8V TO 50V
1
5
V
IN
SHDN
V
CC
V
SW
39µF
63V
6
LT1676
7
FB
8
V
C
2
3
220µH*
MBR160
100µF
10V
EFFICIENCY (%)
+
+
5V
400mA
36.5k
1%
SYNC
2200pF
22k
100pF
12.1k
1%
GND
4
*65T #30 ON MAGNETICS
MPP #55030
1676 F01
Figure 1
U
Efficiency vs V
IN
and I
LOAD
90
80
70
60
50
40 V
IN
= 12V
30
20
1
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
10
I
LOAD
(mA)
1676 TA01
U
U
100
1000
1
LT1676
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 ........................................................ 60V
Switch Voltage ......................................................... 60V
SHDN, SYNC Pin Voltage ........................................... 7V
V
CC
Pin Voltage ....................................................... 30V
FB Pin Voltage ........................................................... 3V
Operating Junction Temperature Range
LT1676C ................................................ 0°C to 125°C
LT1676I ............................................ – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1676CN8
LT1676CS8
LT1676IN8
LT1676IS8
S8 PART MARKING
1676
1676I
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
= 48V, 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
4.0
5.0
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
3.2
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
∆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
LT1676
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
IN
= 48V, V
SW
open, V
CC
= 5V, V
C
= 1.4V unless otherwise noted.
SYMBOL
Timing
f
Switching Frequency
q
PARAMETER
CONDITIONS
MIN
90
85
85
TYP
100
90
300
1.35
0.2
1.6
MAX
110
115
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
High dV/dt Mode, R
L
= 50Ω (Note 4)
Boost Operation
q
q
q
1.5
130
40
0.5
2.2
250
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 50Ω resistive
load to ground.
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)
U W
100
LT1676
G01
Switch-On Voltage vs
Switch Current
1000
Switch Current Limit vs
Duty Cycle
T
A
= 25°C
800
600
125°C
400
200
125
0
100
200 300 400 500 600
SWITCH CURRENT (mA)
700
0
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1676 G03
1676 G02
3
LT1676
TYPICAL PERFORMANCE CHARACTERISTICS
SHDN Pin Shutdown Threshold
vs Temperature
900
SHDN PIN INPUT CURRENT (µA)
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)
106
104
102
100
98
96
94
–50 –25
2.25
SWITCH MINIMUM ON-TIME (ns)
SWITCHING FREQUENCY (kHz)
0
25
50
75
TEMPERATURE (°C)
1676 G07
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)
4
U W
100
LT1676
G04
SHDN Pin Input Current
vs Voltage
5
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
1676 G05
1.22
–20
125
1.20
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
LT1676
G06
Minimum Synchronization Voltage
vs Temperature
600
500
400
300
200
100
Switch Minimum On-Time
vs Temperature
V
IN
= 48V
R
L
= 50Ω
FB =
2.00
1.75
1.50
1.25
1.00
0.75
–50 –25
100
125
0
25
50
75
100
125
0
–50 –25
TEMPERATURE (°C)
1676 G08
50
25
75
0
TEMPERATURE (°C)
100
125
1676 G09
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
100
125
–150
1.0
1.1
1.3
1.4
1.2
FB PIN VOLTAGE (V)
1.5
1676 G11
400
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
LT1676
G10
LT1676
G12
LT1676
PIN FUNCTIONS
SHDN (Pin 1):
When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regula-
tor and reduces V
IN
input current to a few tens of micro-
amperes (shutdown mode).
When this pin is held above the shutdown mode thresh-
old, but below the lockout threshold, the part will be
operational with the exception that output switching
action will be inhibited (lockout mode). A user-adjustable
undervoltage 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 an 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 that meets the input
ripple current requirements is needed here. (See Applica-
tions 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
10% and 90% duty cycle. The sync function is internally
disabled if the FB pin voltage is low enough to cause
oscillator slowdown. If unused, this pin should be grounded.
FB (Pin 7):
This is the inverting input to the feedback
amplifier. The noninverting input of this amplifier is inter-
nally tied to the 1.24V reference. This pin also slows down
the frequency of the internal oscillator when its voltage is
abnormally low, e.g., 2/3 of normal or less. This feature
helps maintain proper short-circuit protection.
V
C
(Pin 8):
This is the control voltage pin which is the
output of the feedback amplifier and the input of the
current comparator. Frequency compensation of the over-
all loop is effected by placing a capacitor, (or in most cases
a series RC combination) between this node and ground.
TIMING DIAGRAMS
High dV/dt Mode
V
IN
V
SW
0
SWDR
SWON
BOOST
SWOFF
1676 TD01
W
U
U
UW
U
Low dV/dt Mode
V
IN
V
SW
0
SWDR
SWON
BOOST
SWOFF
1676 TD02
5
