LT1738
Slew Rate Controlled
Ultralow Noise DC/DC Controller
FEATURES
s
s
s
DESCRIPTIO
s
s
s
s
s
s
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
APPLICATIO S
s
s
s
s
s
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
The LT
®
1738 is a switching regulator controller designed
to lower conducted and radiated electromagnetic interfer-
ence (EMI). Ultralow noise and EMI are achieved by
controlling the voltage and current slew rates of an exter-
nal 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 opti-
mized 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 switch-
ing harmonics.
Protection features include gate drive lockout for low V
IN
,
soft-start, output current limit, short-circuit current limit-
ing, gate drive overvoltage clamp and input supply
undervoltage lockout.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
5V
V
IN
P3
22µH
Ultralow Noise 5V to 12V Converter
MBRD620
4
×
150µF
OSCON
17
V
IN
14
5
6
1.3nF
7
16.9k
25k
25k
3.3k
3.3k
8
16
15
12
SHDN
V5
SYNC
C
T
LT1738
R
T
R
VSL
R
CSL
V
C
SS
13
10nF
GND
11
CS
4
25mΩ
PGND
FB
NFB
10
20
21.5k
9
GATE
1
Si9426
3
GCL
CAP
2
B
10µH
150µF
OSCON
OPTIONAL
5pF
A
+
+
12V
1A
+
100µF
POINT A
ON SCHEMATIC
500µV/DIV
POINT B
ON SCHEMATIC
50mV/DIV
22nF
0.22µF
1.5k
2.5k
1738 TA01
U
12V Output Noise
(Bandwidth = 100MHz)
400µV
P-P
5µs/DIV
1738 TA01a
U
U
1738fa
1
LT1738
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GATE
CAP
GCL
CS
V5
SYNC
C
T
R
T
FB
1
2
3
4
5
6
7
8
9
20 PGND
19 NC
18 NC
17 V
IN
16 R
VSL
15 R
CSL
14 SHDN
13 SS
12 V
C
11 GND
Supply Voltage (V
IN
) ................................................ 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
ORDER PART
NUMBER
LT1738EG
LT1738IG
NFB 10
G PACKAGE
20-LEAD PLASTIC SSOP
T
JMAX
= 150°C,
θ
JA
= 110°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
C
= 0.9V, V
FB
= V
REF
, R
VSL
, R
CSL
= 16.9k, R
T
= 16.9k and
other pins open unless otherwise noted.
SYMBOL
V
REF
I
FB
FB
REG
V
NFR
I
NFR
NFB
REG
g
m
I
ESK
I
ESRC
V
CLH
V
CLL
A
V
FB
OV
I
SS
PARAMETER
Reference Voltage
Feedback Input Current
Reference Voltage Line Regulation
Negative Feedback Reference Voltage
Negative Feedback Input Current
Negative Feedback Reference Voltage Line Regulation
Error Amplifier Transconductance
Error Amp Sink Current
Error Amp Source Current
Error Amp Clamp Voltage
Error Amp Clamp Voltage
Error Amplifier Voltage Gain
FB Overvoltage Shutdown
Soft-Start Charge Current
Outputs Drivers Disabled
V
SS
= 1V
CONDITIONS
Measured at Feedback Pin
V
FB
= V
REF
2.7V
≤
V
IN
≤
20V
Measured at Negative Feedback Pin
with Feedback Pin Open
V
NFB
= V
NFR
2.7V
≤
V
IN
≤
20V
∆I
C
=
±50µA
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
1.235
TYP
1.250
250
0.012
MAX
1.265
1000
0.03
–2.45
UNITS
V
nA
%/V
V
µA
Error Amplifiers
–2.56
–37
1100
700
120
120
– 2.50
– 25
0.009
1500
200
200
1.27
0.12
0.03
2200
2500
350
350
µmho
µmho
µA
µA
V
V
V/V
V
V
FB
= V
REF
+ 150mV, V
C
= 0.9V
V
FB
= V
REF
– 150mV, V
C
= 0.9V
High Clamp, V
FB
= 1V
Low Clamp, V
FB
= 1.5V
q
q
180
250
1.47
9.0
12
1738fa
2
U
%/V
µA
W
U
U
W W
W
LT1738
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
C
= 0.9V, V
FB
= V
REF
, R
VSL
, R
CSL
= 16.9k, R
T
= 16.9k and
other pins open unless otherwise noted.
SYMBOL
f
MAX
f
SYNC
V
SYNC
R
SYNC
Gate Drive
DC
MAX
VG
ON
VG
OFF
IG
SO
IG
SK
V
INUVLO
t
IBL
V
SENSE
V
SENSEF
V
SLEWR
V
SLEWF
VI
SLEWR
VI
SLEWF
V
INMIN
I
VIN
V
SHDN
∆V
SHDN
I
SHDN
V5
IV5
SC
Maximum Switch Duty Cycle
Gate On Voltage
Gate Off Voltage
Max Gate Source Current
Max Gate Sink Current
Gate Drive Undervoltage Lockout (Note 5)
Switch Current Limit Blanking Time
Sense Voltage Shutdown Voltage
Sense Voltage Fault Threshold
Output Voltage Slew Rising Edge
Output Voltage Slew Falling Edge
Output Current Slew Rising Edge (CS pin V)
Output Current Slew Falling Edge (CS pin V)
Minimum Input Voltage (Note 4)
Supply Current (Note 3)
Shutdown Turn-On Threshold
Shutdown Turn-On Voltage Hysteresis
Shutdown Input Current Hysteresis
5V Reference Voltage
5V Reference Short-Circuit Current
6.5V
≤
V
IN
≤
20V, IV5 = 5mA
6.5V
≤
V
IN
≤
20V, IV5 = – 5mA
V
IN
= 6.5V Source
V
IN
= 6.5V Sink
R
VSL
= R
CSL
= 17k
R
VSL
= R
CSL
= 17k
R
VSL
= R
CSL
= 17k
R
VSL
= R
CSL
= 17k
V
GCL
= V
IN
R
VSL
= R
CSL
= 17k
R
VSL
= R
CSL
= 17k
V
IN
= 12
V
IN
= 20
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
PARAMETER
Max Switch Frequency
Synchronization Frequency Range
SYNC Pin Input Threshold
SYNC Pin Input Resistance
CONDITIONS
MIN
TYP
250
MAX
UNITS
kHz
kHz
Oscillator and Sync
Oscillator Frequency = 250kHz
q
290
0.7
1.4
40
2
kΩ
%
10.7
8.1
0.35
V
V
V
A
A
R
VSL
= R
CSL
= 3.9k,
Osc Frequency = 25kHz
V
IN
= 12, GCL = 12
V
IN
= 12, GCL = 8
V
IN
= 12V
V
IN
= 12V
V
IN
= 12V
V
GCL
= 6.5V
q
90
10
7.6
0.3
0.3
93.5
10.4
7.9
0.2
7.3
100
7.5
V
ns
Current Sense
V
C
Pulled Low
q
86
103
220
26
19
2.1
2.1
2.55
12
35
120
300
mV
mV
V/µs
V/µs
V/µs
V/µs
Slew Control for the Following Slew Tests See Test Circuit in Figure 1b
Supply and Protection
3.6
40
55
1.48
180
35
5.20
5.15
V
mA
mA
V
mV
µA
V
V
mA
mA
1.31
50
10
4.85
4.80
10
–10
1.39
110
24
5
5
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
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
IN
is greater than max value.
1738fa
3
LT1738
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Voltage and Input
Current vs Temperature
1.260
1.258
1.256
750
NEGATIVE FEEDBACK VOLTAGE (V)
700
650
600
550
500
450
400
350
300
0
250
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G01
FEEDBACK VOLTAGE (V)
1.254
1.252
1.250
1.248
1.246
1.244
1.242
1.240
–50 –25
Feedback Overvoltage Shutdown
vs Temperature
1.70
1.65
1.60
FEEDBACK VOLTAGE (V)
1.55
1.50
1.45
1.40
1.35
1.30
1.25
1.20
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G03
TRANSCONDUCTANCE (µmho)
1600
1500
1400
1300
1200
1100
1000
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G04
CURRENT (µA)
V
C
Pin Threshold and Clamp
Voltage vs Temperature
1.4
1.2
V
C
PIN VOLTAGE (V)
1.0
0.8
0.6
0.4
0.2
0
–50 –25
THRESHOLD
CLAMP
220
CS PIN VOLTAGE (mV)
200
180
160
140
120
240
SHDN PIN VOLTAGE (V)
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G06
4
U W
Negative Feedback Voltage and
Input Current vs Temperature
2.480
2.485
2.490
2.495
2.500
2.505
2.510
2.515
2.520
–50 –25
0
3.2
3.0
NFB INPUT CURRENT (µA)
2.8
2.6
2.4
2.2
2.0
1.8
1.6
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G02
Error Amp Transconductance vs
Temperature
2000
1900
1800
1700
500
400
300
200
100
0
–100
–200
–300
–400
CS Pin Trip Voltage and CS Fault
Voltage vs Temperature
1.50
FB INPUT CURRENT (nA)
FAULT
TRIP
Error Amp Output Current vs
Feedback Pin Voltage from Nominal
–40°C
25°C
125°C
–500
–400 –300 –200 –100 0 100 200 300 400
FEEDBACK PIN VOLTAGE FROM NOMINAL (mV)
1738 G05
SHDN Pin On and Off Thresholds
vs Temperature
1.45
ON
1.40
1.35
1.30
100
80
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G07
OFF
1.25
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G08
1738fa
LT1738
TYPICAL PERFOR A CE CHARACTERISTICS
SHDN Pin Hysteresis Current vs
Temperature
27
25
SHDN PIN CURRENT (µA)
23
21
19
17
15
–50 –25
V
IN
CURRENT (mA)
15
14
13
12
V
C
PIN VOLTAGE (V)
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G09
Slope Compensation
110
GATE DRIVE A/B PIN VOLTAGE (V)
PERCENT OF MAX CS VOLTAGE
100
90
80
70
60
50
V
C
PIN = 0.9V
T
A
= 25°C
10.7
10.6
10.5
10.4
10.3
10.2
10.1
10.0
9.90
9.80
0
20
40
60
DUTY CYCLE (%)
80
100
1738 G12
GCL = 12V
6.3
6.2
V
IN
= 12V
NO LOAD
6.1
6.0
5.9
GATE DRIVE A/B PIN VOLTAGE (V)
Gate Drive Undervoltage Lockout
Voltage vs Temperature
7.3
7.2
7.1
SS PIN CURRENT (µA)
V
IN
PIN VOLTAGE (V)
7.0
6.9
6.8
6.7
6.6
6.5
6.4
6.3
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G15
GCL = 6V
8.9
8.7
8.5
8.3
8.1
7.9
7.7
7.5
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G16
V5 PIN VOLTAGE (V)
U W
V
IN
Current vs Temperature
18
17
16
V
IN
= 20 R
VSL
, R
CSL
= 17k
V
IN
= 12 R
VSL
, R
CSL
= 4.85k
CS Pin to V
C
Pin Transfer
Function
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
20
40
60
80
CS PIN VOLTAGE (mV)
100
120
1738 G11
T
A
= 25°C
V
IN
= 12 R
VSL
, R
CSL
= 17k
11 USING LOAD OF FIGURE 1A
f
OSC
= 120kHz
10
–50 –25 0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G10
Gate Drive High Voltage vs
Temperature
6.5
6.4
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Gate Drive Low Voltage vs
Temperature
V
IN
= 12V
NO LOAD
GCL = 6V
5.8
5.7
5.6
9.70
–50 –25
0
5.5
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G13
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1738 G14
Soft-Start Current vs Temperature
9.5
9.3
9.1
SS VOLTAGE = 0.9V
5.06
5.04
5.02
5.00
5.08
V5 Voltage vs Load Current
T = 125°C
T = 25°C
T = –40°C
4.98
4.96
–15
–10
–5
0
5
LOAD CURRENT (mA)
10
15
1738 G17
1738fa
5
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