ANALOG DEVICES LT1676CS8PBF Datasheet

FEATURES

■

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

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APPLICATIO S

■

Automotive DC/DC Converters

■

Telecom 48V Step-Down Converters

■

Cellular Phone Battery Charger Accessories

■

IEEE 1394 Step-Down Converters

LT1676

Wide Input Range,

High Efficiency, Step-Down

Switching Regulator

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DESCRIPTIO

The LT®1676 is a wide input range, high efficiency Buck
(step-down) switching regulator. The monolithic die in­cludes 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 rejec­tion and short-circuit protection.

The LT1676 contains several features to enhance effi­ciency. The internal control circuitry is normally powered
via the VCC pin, thereby minimizing power drawn directly
from the VIN supply (see Applications Information). The
action of the LT1676 switch circuitry is also load depen­dent. 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.

TYPICAL APPLICATIO

V

IN

8V TO 50V

1

+

39µF
63V

6

SHDN

SYNC

V

IN

LT1676

GND

5

V

CC

V

SW

FB
V

C

4

U

2
3

7
8

2200pF

22k

Figure 1

220µH*

+

MBR160

100pF

*65T #30 ON MAGNETICS
MPP #55030

100µF
10V

The available SO-8 package and 100kHz switching fre­quency allow for minimal PC board area requirements.

, LTC and LT are registered trademarks of Linear Technology Corporation.

36.5k
1%

12.1k
1%

1676 F01

5V
400mA

Efficiency vs VIN and I

90

80

70

60

50

EFFICIENCY (%)

VIN = 12V

40

= 24V

V

IN

30

V

= 36V

IN

= 48V

V

IN

20

1

10 100 1000

I

(mA)

LOAD

LOAD

1676 TA01

1

LT1676

WW

W

ABSOLUTE MAXIMUM RA TIN GS

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U

W

PACKAGE/ORDER INFORMA TION

(Note 1)

Supply Voltage ........................................................ 60V

Switch Voltage......................................................... 60V

SHDN, SYNC Pin Voltage........................................... 7V

VCC 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

TOP VIEW

1

SHDN

V

2

CC

V

3

SW

GND

4

N8 PACKAGE
8-LEAD PDIP

T

= 125°C, θJA = 130°C/W (N8)

JMAX

= 125°C, θJA = 110°C/W (S8)

T

JMAX

Consult factory for Military grade parts.

8-LEAD PLASTIC SO

V

8

C

FB

7

SYNC

6

V

5

IN

S8 PACKAGE

ORDER PART

NUMBER

LT1676CN8
LT1676CS8
LT1676IN8
LT1676IS8

S8 PART MARKING

1676
1676I

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.

VIN = 48V, VSW open, VCC = 5V, VC = 1.4V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Power Supplies

V

IN(MIN)

I

VIN

I

VCC

V

VCC

Feedback Amplifier

V

REF

I

IN

g

m

I

, I

SRC

V

CL

Output Switch

V

ON

I

LIM

Current Amplifier

Minimum Input Voltage 6.7 7.0 V

● 7.4 V

VIN Supply Current VC = 0V 620 800 µA

● 900 µA

VCC Supply Current VC = 0V 3.2 4.0 mA

● 5.0 mA

VCC Dropout Voltage (Note 2) ● 2.8 3.1 V
Shutdown Mode I

Reference Voltage 1.225 1.240 1.255 V

FB Pin Input Bias Current 600 1500 nA
Feedback Amplifier Transconductance ∆lc = ±10µA 400 650 1000 µmho

Feedback Amplifier Source or Sink Current 60 100 170 µA

SNK

Feedback Amplifier Clamp Voltage 2.0 V
Reference Voltage Line Regulation 12V ≤ VIN ≤ 60V ● 0.01 %/V
Voltage Gain 200 600 V/V

Output Switch On Voltage ISW = 0.5A 1.0 1.5 V
Switch Current Limit (Note 3) ● 0.55 0.70 1.0 A

Control Pin Threshold Duty Cycle = 0% 0.9 1.1 1.25 V
Control Voltage to Switch Transconductance 2 A/V

VIN

V

= 0V 30 50 µA

SHDN

● 75 µA

● 1.215 1.265 V

● 200 1500 µmho

● 45 220 µA

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2

LT1676

DUTY CYCLE (%)

0

SWITCH CURRENT LIMIT (mA)

1000

800

600

400

200

0

80

1676 G03

2010 30 50 70 90

40

60

100

TA = 25°C

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.

VIN = 48V, VSW open, VCC = 5V, VC = 1.4V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Timing

f Switching Frequency 90 100 110 kHz

● 85 115 kHz

Maximum Switch Duty Cycle ● 85 90 %

t

ON(MIN)

Boost Operation

Sync Function

SHDN Pin Function

V

SHDN

I

SHDN

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

Minimum Switch On Time High dV/dt Mode, RL = 50Ω (Note 4) 300 ns

VC Pin Boost Threshold 1.35 V
dV/dt Below Threshold 0.2 V/ns
dV/dt Above Threshold 1.6 V/ns

Minimum Sync Amplitude ● 1.5 2.2 V
Synchronization Range ● 130 250 kHz
SYNC Pin Input R 40 kΩ

Shutdown Mode Threshold 0.5 V

● 0.2 0.8 V

Upper Lockout Threshold Switching Action On 1.260 V
Lower Lockout Threshold Switching Action Off 1.245 V
Shutdown Pin Current V

= 0V 12 20 µA

SHDN

V

= 1.25V 2.5 10 µA

SHDN

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

CC

.

application.
Note 4: Minimum switch on time is production tested with a 50Ω resistive

load to ground.

TYPICAL PERFORMANCE CHAR ACTERISTICS

Minimum Input Voltage vs
Temperature

7.4

7.2

7.0

6.8

6.6

INPUT VOLTAGE (V)

6.4

6.2

6.0
–50

–25 0

50 100 125

25 75

TEMPERATURE (°C)

LT1676 G01

UW

1.50

1.25

1.00

0.75

0.50

SWITCH VOLTAGE (V)

0.25

0

Switch-On Voltage vs
Switch Current

25°C

400 600 700

300 500

0

–55°C

125°C

100 200

SWITCH CURRENT (mA)

Switch Current Limit vs
Duty Cycle

1676 G02

3

LT1676

UW

TYPICAL PERFORMANCE CHAR ACTERISTICS

SHDN Pin Shutdown Threshold
vs Temperature

900

800

700

600

500

400

SHDN PIN VOLTAGE (mV)

300

200

–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

Switching Frequency
vs Temperature

106

104

102

LT1676 G04

SHDN Pin Input Current
vs Voltage

5

0

–5

–10

–15

SHDN PIN INPUT CURRENT (µA)

–20

1

0

SHDN PIN VOLTAGE (V)

3

2

Minimum Synchronization Voltage
vs Temperature

2.25

2.00

1.75

25°C
–55°C
125°C

4

1676 G05

SHDN Pin Lockout Thresholds
vs Temperature

1.30

1.28
UPPER THRESHOLD

1.26

LOWER THRESHOLD

1.24

SHDN PIN VOLTAGE (V)

1.22

1.20

–50

5

–25 0

TEMPERATURE (°C)

50 100 125

25 75

LT1676 G06

Switch Minimum On-Time
vs Temperature

600

V

= 48V

IN

= 50Ω

R

L

500

FB =

400

100

98

SWITCHING FREQUENCY (kHz)

96

94

–50 25 75

–25 0

TEMPERATURE (°C)

50 100 125

VC Pin Switching Threshold,
Boost Threshold, Clamp Voltage
vs Temperature

2.2

2.0

1.8

1.6

1.4

PIN VOLTAGE (V)

C

V

1.2

1.0

0.8
–50

–25 0

25 75

TEMPERATURE (°C)

CLAMP

VOLTAGE

THRESHOLD

SWITCHING

THRESHOLD

50 100 125

BOOST

1676 G07

LT1676 G10

1.50

1.25

1.00

MINIMUM SYNCHRONIZATION VOLTAGE (V)

0.75
–50 25 75

–25 0

TEMPERATURE (°C)

50 100 125

Feedback Amplifier Output
Current vs FB Pin Voltage

100

50

0

–50

–100

FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)

–150

1.0

1.1

1.2

FB PIN VOLTAGE (V)

1.3

25°C
–55°C
125°C

1.4

1676 G08

1676 G11

1.5

300

200

100

SWITCH MINIMUM ON-TIME (ns)

0

–50 25 75

–25 0

TEMPERATURE (°C)

50 100 125

Error Amplifier Transconductance
vs Temperature

750

700

650

600

550

500

TRANSCONDUCTANCE (µmho)

450

400

–50

–25 0

TEMPERATURE (°C)

50 100 125

25 75

1676 G09

LT1676 G12

4

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PIN FUNCTIONS

LT1676

SHDN (Pin 1):

When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regula­tor and reduces VIN input current to a few tens of micro­amperes (shutdown mode).

When this pin is held above the shutdown mode thresh­old, 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 VIN. This action is
logically “ANDed” with the internal UVLO, set at nominally

6.7V, such that minimum VIN 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 VSW node, so a small
capacitor to ground, typically 100pF or so is recom­mended when the pin is left “open.”

VCC (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 VIN. If the output capacitor is located more
than an inch from the VCC pin, a separate 0.1µF bypass
capacitor to ground may be required right at the pin.

VSW (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.
VIN (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 VCC pin is left
open. A high quality bypass capacitor that 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
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 inter­nally 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.

VC (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 over­all loop is effected by placing a capacitor, (or in most cases
a series RC combination) between this node and ground.

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TIMING DIAGRAMS

High dV/dt Mode Low dV/dt Mode

V

IN

V

SW

0

SWDR

SWON

BOOST

SWOFF

1676 TD01

V

V

SW

SWDR

SWON

BOOST

SWOFF

IN

0

1676 TD02

5