LINEAR TECHNOLOGY LT1738 Technical data

Ultralow Noise DC/DC Controller

FEATURES

■

Greatly Reduced Conducted and Radiated EMI

■

Low Switching Harmonic Content

■

Independent Control of Output Switch Voltage and
Current Slew Rates

■

Greatly Reduced Need for External Filters

■

Single N-Channel MOSFET Driver

■

20kHz to 250kHz Oscillator Frequency

■

Easily Synchronized to External Clock

■

Regulates Positive and Negative Voltages

■

Easier Layout Than with Conventional Switchers

U

APPLICATIO S

■

Power Supplies for Noise Sensitive Communication
Equipment

■

EMI Compliant Offline Power Supplies

■

Precision Instrumentation Systems

■

Isolated Supplies for Industrial Automation

■

Medical Instruments

■

Data Acquisition Systems

LT1738

Slew Rate Controlled

U

DESCRIPTIO

The LT®1738 is a switching regulator controller designed
to lower conducted and radiated electromagnetic interfer­ence (EMI). Ultralow noise and EMI are achieved by
controlling the voltage and current slew rates of an exter­nal N-channel MOSFET switch. Current and voltage slew
rates can be independently set to optimize harmonic
content of the switching waveforms vs efficiency. The
LT1738 can reduce high frequency harmonic power by as
much as 40dB with only minor losses in efficiency.

The LT1738 utilizes a current mode architecture opti­mized for single switch topologies such as boost, flyback
and Cuk. The IC includes gate drive and all necessary
oscillator, control and protection circuitry. Unique error
amp circuitry can regulate both positive and negative
voltages. The internal oscillator may be synchronized to
an external clock for more accurate placement of switch­ing harmonics.

Protection features include gate drive lockout for low VIN,
soft-start, output current limit, short-circuit current limit­ing, gate drive overvoltage clamp and input supply
undervoltage lockout.

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

TYPICAL APPLICATIO

Ultralow Noise 5V to 12V Converter

GCL

GATE

PGND

NFB

CAP

22µH

3

5pF

1

4

CS

20

9

FB

101113

5V

V

IN

100µF

1.3nF

25k

25k

0.22µF

+

14 2

16.9k

3.3k

16

3.3k

15

22nF

12

1.5k

10nF

P3

17

V

IN

SHDN

5

V5

6

SYNC

7

C

T

LT1738

8

R

T

R

VSL

R

CSL

V

C

GND

SS

MBRD620

Si9426

25mΩ

U

4 × 150µF

OSCON

21.5k

2.5k

12V Output Noise

10µH

+

150µF

OSCON

OPTIONAL

AB

12V
1A

+

POINT A

ON SCHEMATIC

500µV/DIV

POINT B

ON SCHEMATIC

50mV/DIV

1738 TA01

(Bandwidth = 100MHz)

5µs/DIV

1738 TA01a

400µV

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1

LT1738

1
2
3
4
5
6
7
8
9

10

TOP VIEW

G PACKAGE

20-LEAD PLASTIC SSOP

20
19
18
17
16
15
14
13
12
11

GATE

CAP
GCL

CS
V5

SYNC

C

T

R

T

FB

NFB

PGND
NC
NC
V

IN

R

VSL

R

CSL

SHDN
SS
V

C

GND

WW

W

ABSOLUTE AXI U RATI GS

U

PACKAGE/ORDER I FOR ATIO

(Note 1)

Supply Voltage (VIN)................................................ 20V

Gate Drive Current .....................................Internal Limit

V5 Current .................................................Internal Limit

SHDN Pin Voltage.................................................... 20V

Feedback Pin Voltage (Trans. 10ms) ...................... ±10V

Feedback Pin Current............................................ 10mA

Negative Feedback Pin Voltage (Trans. 10ms)........ ±10V

CS Pin.......................................................................... 5V

GCL Pin ..................................................................... 16V

SS Pin.......................................................................... 3V

Operating Junction Temperature Range

(Note 3) ............................................ –40°C to 125°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VC = 0.9V, VFB = V
other pins open unless otherwise noted.

The ● denotes the specifications which apply over the full operating

Consult LTC Marketing for parts specified with wider operating temperature ranges.

T

= 150°C, θJA = 110°C/W

JMAX

REF

, R

VSL

UUW

ORDER PART

NUMBER

LT1738EG
LT1738IG

, R

= 16.9k, RT = 16.9k and

CSL

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Error Amplifiers

V

REF

I

FB

FB

REG

V

NFR

I

NFR

NFB
g

m

I

ESK

I

ESRC

V

CLH

V

CLL

A

V

FB

OV

I

SS

Reference Voltage Measured at Feedback Pin ● 1.235 1.250 1.265 V
Feedback Input Current VFB = V
Reference Voltage Line Regulation 2.7V ≤ VIN ≤ 20V ● 0.012 0.03 %/V
Negative Feedback Reference Voltage Measured at Negative Feedback Pin ● –2.56 –2.50 –2.45 V

Negative Feedback Input Current V
Negative Feedback Reference Voltage Line Regulation 2.7V ≤ VIN ≤ 20V ● 0.009 0.03 %/V

REG

Error Amplifier Transconductance ∆IC = ±50µA 1100 1500 2200 µmho

Error Amp Sink Current VFB = V
Error Amp Source Current VFB = V
Error Amp Clamp Voltage High Clamp, VFB = 1V 1.27 V
Error Amp Clamp Voltage Low Clamp, VFB = 1.5V 0.12 V
Error Amplifier Voltage Gain 180 250 V/V
FB Overvoltage Shutdown Outputs Drivers Disabled 1.47 V
Soft-Start Charge Current VSS = 1V 9.0 12 µA

REF

with Feedback Pin Open

= V

NFB

NFR

+ 150mV, VC = 0.9V ● 120 200 350 µA

REF

– 150mV, VC = 0.9V ● 120 200 350 µA

REF

● 250 1000 nA

–37 –25 µA

● 700 2500 µmho

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LT1738

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VC = 0.9V, VFB = V

The ● denotes the specifications which apply over the full operating

, R

, R

REF

VSL

= 16.9k, RT = 16.9k and

CSL

other pins open unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Oscillator and Sync

f

MAX

f

SYNC

V

SYNC

R

SYNC

Gate Drive

DC

MAX

VG

ON

VG

OFF

IG

SO

IG

SK

V

INUVLO

Current Sense

t

IBL

V

SENSE

V

SENSEF

Slew Control for the Following Slew Tests See Test Circuit in Figure 1b

V

SLEWR

V

SLEWF

VI

SLEWR

VI

SLEWF

Supply and Protection

V

INMIN

I

VIN

V

SHDN

∆V

SHDN

I

SHDN

V5 5V Reference Voltage 6.5V ≤ VIN ≤ 20V, IV5 = 5mA 4.85 5 5.20 V

IV5

SC

Note 1: Absolute Maximum Ratings are those values beyond which the
life of a device may be impaired.

Note 2: Supply current specification includes loads on each gate as in
Figure 1a. Actual supply currents vary with operating frequency, operating
voltages, V5 load, slew rates and type of external FET.

Note 3: The LT1738E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications from –40°C to 125°C are assured by

Max Switch Frequency 250 kHz
Synchronization Frequency Range Oscillator Frequency = 250kHz 290 kHz
SYNC Pin Input Threshold ● 0.7 1.4 2
SYNC Pin Input Resistance 40 kΩ

Maximum Switch Duty Cycle R

VSL

= R

= 3.9k, ● 90 93.5 %

CSL

Osc Frequency = 25kHz

Gate On Voltage VIN = 12, GCL = 12 10 10.4 10.7 V

V

= 12, GCL = 8 7.6 7.9 8.1 V

IN

Gate Off Voltage VIN = 12V 0.2 0.35 V
Max Gate Source Current VIN = 12V 0.3 A
Max Gate Sink Current VIN = 12V 0.3 A
Gate Drive Undervoltage Lockout (Note 5) V

= 6.5V 7.3 7.5 V

GCL

Switch Current Limit Blanking Time 100 ns
Sense Voltage Shutdown Voltage VC Pulled Low ● 86 103 120 mV
Sense Voltage Fault Threshold 220 300 mV

Output Voltage Slew Rising Edge R
Output Voltage Slew Falling Edge R
Output Current Slew Rising Edge (CS pin V) R
Output Current Slew Falling Edge (CS pin V) R

Minimum Input Voltage (Note 4) V
Supply Current (Note 3) R

R

VSL

VSL

VSL

VSL

GCL

VSL
VSL

= R

= 17k 26 V/µs

CSL

= R

= 17k 19 V/µs

CSL

= R

= 17k 2.1 V/µs

CSL

= R

= 17k 2.1 V/µs

CSL

= V

IN

= R

= 17k V

CSL

= R

= 17k V

CSL

= 12 ● 12 40 mA

IN

= 20 35 55 mA

IN

● 2.55 3.6 V

Shutdown Turn-On Threshold ● 1.31 1.39 1.48 V
Shutdown Turn-On Voltage Hysteresis ● 50 110 180 mV
Shutdown Input Current Hysteresis ● 10 24 35 µA

6.5V ≤ V

≤ 20V, IV5 = –5mA 4.80 5 5.15 V

IN

5V Reference Short-Circuit Current VIN = 6.5V Source 10 mA

= 6.5V Sink –10 mA

V

IN

design, characterization and correlation with statistical process
controls.The LT1738I is guaranteed and tested to meet performance
specifications from – 40°C to 125°C.

Note 4: Output gate drive is enabled at this voltage. The GCL voltage will
also determine driver activity.

Note 5: Gate drive is ensured to be on when V

is greater than max value.

IN

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3

LT1738

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Feedback Voltage and Input
Current vs Temperature

1.260

1.258

1.256

1.254

1.252

1.250

1.248

1.246

FEEDBACK VOLTAGE (V)

1.244

1.242

1.240
–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

Feedback Overvoltage Shutdown
vs Temperature

1.70

1.65

1.60

1.55

1.50

1.45

1.40

1.35

FEEDBACK VOLTAGE (V)

1.30

1.25

1.20
–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

1738 G03

750
700
650

FB INPUT CURRENT (nA)

600
550
500
450
400
350
300
250

1738 G01

Error Amp Transconductance vs
Temperature

2000
1900
1800
1700
1600
1500
1400
1300
1200

TRANSCONDUCTANCE (µmho)

1100
1000

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

Negative Feedback Voltage and
Input Current vs Temperature

2.480

2.485

2.490

2.495

2.500

2.505

2.510

NEGATIVE FEEDBACK VOLTAGE (V)

2.515

2.520
–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

Error Amp Output Current vs
Feedback Pin Voltage from Nominal

500
400
300
200
100

0

–100

CURRENT (µA)

–200
–300
–400
–500

–400 –300 –200 –100 0 100 200 300 400

FEEDBACK PIN VOLTAGE FROM NOMINAL (mV)

1738 G04

1738 G02

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

–40°C

125°C

NFB INPUT CURRENT (µA)

25°C

1738 G05

VC Pin Threshold and Clamp
Voltage vs Temperature

1.4

1.2

1.0

0.8

0.6

PIN VOLTAGE (V)

C

V

0.4

0.2

0

–50 –25 0 25 50 75 100 125 150

CLAMP

THRESHOLD

TEMPERATURE (°C)

4

1738 G06

CS Pin Trip Voltage and CS Fault
Voltage vs Temperature

240

220

200

180

160

140

CS PIN VOLTAGE (mV)

120

100

80

–50 –25 0 25 50 75 100 125 150

FAULT

TRIP

TEMPERATURE (°C)

1738 G07

SHDN Pin On and Off Thresholds
vs Temperature

1.50

1.45

1.40

1.35

SHDN PIN VOLTAGE (V)

1.30

1.25
–50 –25 0 25 50 75 100 125 150

ON

OFF

TEMPERATURE (°C)

1738 G08

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TYPICAL PERFOR A CE CHARACTERISTICS

LT1738

SHDN Pin Hysteresis Current vs
Temperature VIN Current vs Temperature

27

25

23

21

19

SHDN PIN CURRENT (µA)

17

15

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

1738 G09

18

VIN = 20 R

VSL

= 120kHz

0

TEMPERATURE (°C)

VIN = 12 R

VSL

, R

CSL

17

16

15

14

CURRENT (mA)

VIN = 12 R

13

IN

V

12

11

USING LOAD OF FIGURE 1A
f

OSC

10

–50 –25 25 75 125

Gate Drive High Voltage vs

Slope Compensation

110

100

90

80

70

PERCENT OF MAX CS VOLTAGE

60

50

20 40 60 80 100

0

DUTY CYCLE (%)

VC PIN = 0.9V

= 25°C

T

A

1738 G12

Temperature

10.7

10.6

10.5

10.4

10.3

10.2

10.1

10.0

9.90

GATE DRIVE A/B PIN VOLTAGE (V)

9.80

9.70
–50 –25 0 25 50 75 100 125 150

GCL = 12V

GCL = 6V

TEMPERATURE (°C)

= 17k

50

VSL

, R

, R

CSL

= 17k

CSL

VIN = 12V
NO LOAD

= 4.85k

100

1738 G10

1738 G13

150

CS Pin to VC Pin Transfer
Function

1.6
TA = 25°C

1.4

1.2

1.0

0.8

0.6

PIN VOLTAGE (V)

C

V

0.4

0.2

0

20 40 60 80 100 120

0

Gate Drive Low Voltage vs
Temperature

6.5

0.50

6.4

6.3

6.2

6.1

6.0

5.9

5.8

5.7

5.6

5.5

VIN = 12V

0.45
NO LOAD

0.40

0.35

0.30

0.25

0.20

0.15

0.10

GATE DRIVE A/B PIN VOLTAGE (V)

0.05

0

–50 –25 0 25 50 75 100 125 150

CS PIN VOLTAGE (mV)

1738 G11

TEMPERATURE (°C)

1738 G14

Gate Drive Undervoltage Lockout
Voltage vs Temperature

7.3
GCL = 6V

7.2

7.1

7.0

6.9

6.8

6.7

PIN VOLTAGE (V)

IN

6.6

V

6.5

6.4

6.3

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

1738 G15

Soft-Start Current vs Temperature

9.5
SS VOLTAGE = 0.9V

9.3

9.1

8.9

8.7

8.5

8.3

8.1

SS PIN CURRENT (µA)

7.9

7.7

7.5

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

1738 G16

V5 Voltage vs Load Current

5.08

5.06

5.04

5.02

5.00

V5 PIN VOLTAGE (V)

4.98

4.96
–10 –5 0 51015

–15

T = 125°C

T = 25°C

T = –40°C

LOAD CURRENT (mA)

1738 G17

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5

LT1738

PI FU CTIO S

UUU

Part Supply

V5 (Pin 5): This pin provides a 5V output that can sink or

source 10mA for use by external components. V5 source
current comes from V
must be greater than 6.5V in order for this voltage to be in
regulation. If this pin is used, a small capacitor (<1µF) may
be placed on this pin to reduce noise. This pin can be left
open if not used.

GND (Pin 11): Signal Ground. The internal error amplifier,
negative feedback amplifier, oscillator, slew control cir­cuitry, V5 regulator, current sense and the bandgap refer­ence are referred to this ground. Keep the connection to
this pin, the feedback divider and VC compensation net­work free of large ground currents.

SHDN (Pin 14): The shutdown pin can disable the switcher.
Grounding this pin will disable all internal circuitry.

Increasing SHDN voltage will initially turn on the internal
bandgap regulator. This provides a precision threshold for
the turn on of the rest of the IC. As SHDN increases past

1.39V the internal LDO regulator turns on, enabling the
control and logic circuitry.

24µA of current is sourced out of the pin above the turn on
threshold. This can be used to provide hysteresis for the
shutdown function. The hysteresis voltage will be set by
the Thevenin resistance of the resistor divider driving this
pin times the current sourced out. Above approximately

2.1V the hysteresis current is removed. There is approxi­mately 0.1V of voltage hysteresis on this pin as well.

The pin can be tied high (to V

V

(Pin 17): Input Supply. All supply current for the part

IN

comes from this pin including gate drive and V5 regulator.
Charge current for gate drive can produce current pulses
of hundreds of milliamperes. Bypass this pin with a low
ESR capacitor.

. Sink current goes to GND. V

IN

for instance).

IN

IN

PGND (Pin 20): Power Driver Ground. This ground comes
from the MOSFET gate driver. This pin can have several
hundred milliamperes of current on it when the external
MOSFET is being turned off.

Oscillator

SYNC (Pin 6): The SYNC pin can be used to synchronize

the part to an external clock. The oscillator frequency
should be set close to the external clock frequency.
Synchronizing the clock to an external reference is useful
for creating more stable positioning of the switcher volt­age and current harmonics. This pin can be left open or
tied to ground if not used.

CT (Pin 7): The oscillator capacitor pin is used in conjunc­tion with RT to set the oscillator frequency. For RT = 16.9k:

C

(nf) = 129/f

OSC

RT (Pin 8): A resistor to ground sets the charge and
discharge currents of the oscillator capacitor. The nomi­nal value is 16.9k. It is possible to adjust this resistance
±25% to set oscillator frequency more accurately.

OSC

(kHz)

Gate Drive

GATE (Pin 1): This pin connects to the gate of an external

N-channel MOSFET. This driver is capable of sinking and
sourcing at least 300mA.

The GCL pin sets the upper voltage of the gate drive. The
GATE pin will not be activated until V
voltage as defined by the GCL pin (gate undervoltage
lockout).

The gate drive output has current limit protection to safe
guard against accidental shorts.

GCL (Pin 3): This pin sets the maximum gate voltage to the
GATE pin to the MOSFET gate drive. This pin should be
either tied to a zener, a voltage source, or VIN.

reaches a minimum

IN

When V
undervoltage lockout where the gate driver is driven low.
This, along with gate drive undervoltage lockout, prevents
unpredictable behavior during power up.

is below 2.55V the part will go into supply

IN

6

If the pin is tied to a zener or a voltage source, the
maximum gate drive voltage will be approximately V
– 0.2V. If it is tied to VIN, the maximum gate voltage is
approximately VIN – 1.6.

GCL

1738fa