LT8638S
42V, 10A/12A Peak Synchronous
Step-Down Silent Switcher 2
FEATURES
DESCRIPTION
Silent Switcher®2 Architecture
n Ultralow EMI Emissions on Any PCB
n Eliminates PCB Layout Sensitivity
n Internal Bypass Capacitors Reduce Radiated EMI
n Optional Spread Spectrum Modulation
n High Efficiency at High Frequency
n Up to 96% Efficiency at 1MHz, 12V to 5V
IN
OUT
n Up to 94% Efficiency at 2MHz, 12V to 5V
IN
OUT
n Wide Input Voltage Range: 2.8V to 42V
n 10A Maximum Continuous, 12A Peak Transient Output
n Fast Transient Response with External Compensation
n Low Quiescent Current Burst Mode® Operation
n 90µA I Regulating 12V to 5V
Q
IN
OUT
n Output Ripple < 10mV
P-P
n Reference Accuracy: ±1% Over Temperature
n Fast Minimum Switch On-Time: 25ns
n PolyPhase® Operation: Up to 12 Phases
n Low Dropout Under All Conditions: 45mV at 1A
n Adjustable and Synchronizable: 200kHz to 3MHz
n Output Soft-Start and Power Good
n Safely Tolerates High Reverse Current
n 28-Lead 5mm × 4mm LQFN Package
n AEC-Q100 Qualified for Automotive Applications
The LT®8638S synchronous step-down regulator
features second generation Silent Switcher architecture
designed to minimize EMI emissions while delivering high
efficiency at high switching frequencies. This includes
the integration of input capacitors to optimize all the fast
current loops inside and make it easy to achieve advertised
EMI performance by reducing layout sensitivity. This
performance makes the LT8638S ideal for noise sensitive
applications and environments.
n
APPLICATIONS
Automotive and Industrial Supplies
n General Purpose Step-Down
The fast, clean, low overshoot switching edges enable high
efficiency operation even at high switching frequencies,
leading to a small overall solution size. Peak current mode
control with a 25ns minimum on-time allows high step
down ratios even at high switching frequencies. External
compensation via the VC pin allows for fast transient
response. PolyPhase operation allows multiple LT8638S
regulators to run with interleaving phase shift to provide
more output current.
Burst Mode operation enables low standby current
consumption, forced continuous mode can control
frequency harmonics across the entire output load
range, or spread spectrum operation can further reduce
EMI emissions. Soft-start and tracking functionality is
accessed via the SS pin, and an accurate input voltage
UVLO threshold can be set using the EN/UV pin.
All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
n
TYPICAL APPLICATION
12VIN to 5VOUT Efficiency
5V 10A Step-Down Converter
VIN
BST
EN/UV
VOUT
5V
10A
SW
BIAS
INTVCC
1µF
12.1k
330pF
VC
LT8638S
RT
38.3k
fSW = 1MHz
15pF
100k
4.8
EFFICIENCY
3.2
80
2.4
POWER LOSS
75
70
13.7k
4.0
85
47µF
FB
GND
95
90
0.1µF
1µH
EFFICIENCY (%)
4.7µF
5.6
65
8638S TA01a
1
2
1.6
POWER LOSS (W)
VIN
5.4V TO 42V
100
400kHz, L = 2.2µH 0.8
1MHz, L = 1µH
2MHz, L = 0.68µH
0
3
4
5
6
7
8
9 10
LOAD CURRENT (A)
8638S TA01b
Rev. 0
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1
LT8638S
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
FB
SS
VC
PG
SYNC/MODE
CLKOUT
TOP VIEW
28
27
26
25
24
23
PHMODE
1
BIAS
2
INTVCC
3
BST
4
19 NC
SW
5
18 VIN
SW
6
SW
7
SW
8
22 RT
29
GND
21 EN/UV
30
GND
31
GND
20 GND
17 VIN
32
GND
16 VIN
9
10
11
12
13
14
GND
GND
GND
GND
GND
15 VIN
GND
VIN, EN/UV, PG...........................................................42V
BIAS...........................................................................25V
FB, SS, PHMODE ........................................................4V
SYNC/MODE Voltage ..................................................6V
Operating Junction Temperature Range (Note 2)
LT8638SE........................................... –40°C to 125°C
LT8638SJ........................................... –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Maximum Reflow (Package Body) Temperature...... 260°C
LQFN PACKAGE
28-LEAD (5mm × 4mm × 0.94mm)
JEDEC BOARD: θJA = 30°C/W, θJC(TOP) = 14.7°C/W, θJC(PAD) = 2.7°C/W (Note 3)
DEMO BOARD: θJA = 19°C/W, ΨJT = 0.1°C/W
EXPOSED PAD (PINS 29-32) ARE GND, SHOULD BE SOLDERED TO PCB
ORDER INFORMATION
PART NUMBER
LT8638SEV#PBF
LT8638SJV#PBF
PART MARKING*
PAD OR BALL
FINISH
DEVICE
FINISH CODE
PACKAGE
TYPE**
MSL
RATING
Au (RoHS)
8638S
e4
LQFN (Laminate Package
with QFN Footprint)
3
8638S
e4
LQFN (Laminate Package
with QFN Footprint)
3
TEMPERATURE RANGE
(SEE NOTE 2)
–40°C to 125°C
–40°C to 150°C
AUTOMOTIVE PRODUCTS***
LT8638SEV#WPBF
LT8638SJV#WPBF
Au (RoHS)
–40°C to 125°C
–40°C to 150°C
• Contact the factory for parts specified with wider operating temperature
ranges. *Pad or ball finish code is per IPC/JEDEC J-STD-609.
• Recommended LGA and BGA PCB Assembly and Manufacturing
Procedures
• Device temperature grade is identified by a label on the shipping container.
• LGA and BGA Package and Tray Drawings
Parts ending with PBF are RoHS and WEEE compliant. **The LT8638S package has the same dimensions as a standard 5mm × 4mm QFN package.
***Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your
local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for
these models.
2
Rev. 0
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LT8638S
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
Minimum Input Voltage
fSW = 2MHz
VIN Quiescent Current in Shutdown
VEN/UV = 0V, VIN = 12V
VIN Quiescent Current in Sleep
VEN/UV = 2V, VFB > 0.6V, VSYNC = 0V, VBIAS = 0V
MIN
l
l
TYP
MAX
2.6
2.8
UNITS
V
6
9
µA
125
125
195
245
µA
µA
VEN/UV = 2V, VFB > 0.6V, VSYNC = 0V, VBIAS = 5V
20
29
µA
BIAS Quiescent Current in Sleep
VEN/UV = 2V, VFB > 0.6V, VSYNC = 0V, VBIAS = 5V
100
145
µA
Feedback Reference Voltage
VIN = 12V
VIN = 12V
l
0.6
0.6
0.602
0.604
V
V
VIN = 4.0V to 40V, VCC = 1.25V
l
0.004
0.03
%/V
20
nA
1.4
1.75
mS
Feedback Voltage Line Regulation
0.598
0.594
Feedback Pin Input Current
VFB = 0.6V
–20
Error Amp Transconductance
VC = 1.25V
1.05
Error Amp Gain
700
VC Source Current
VFB = 0.4V, VC = 1.25V
320
µA
VC Sink Current
VFB = 0.8V, VC = 1.25V
320
µA
VC Pin to Switch Current Gain
12
A/V
VC Clamp Voltage
2.3
V
45
mA
BIAS Pin Current Consumption
VBIAS = 3.3V, fSW = 2MHz, VIN = 12V
Minimum On-Time
ILOAD = 3A, FCM
l
Minimum Off-Time
Oscillator Frequency
RT = 226k
RT = 38.3k
RT = 16.9k
Top Power NMOS On-Resistance
ISW = 1A
l
l
l
170
0.96
1.85
25
40
ns
80
100
ns
200
1
2
230
1.04
2.15
kHz
MHz
MHz
20
Top Power NMOS Current Limit
l
Bottom Power NMOS On-Resistance
VINTVCC = 3.4V, ISW = 1A
Bottom Power NMOS Current Limit
VINTVCC = 3.4V
SW Leakage Current
VIN = 42V, VSW = 0V, 42V
EN/UV Pin Threshold
EN/UV Rising
17
20
12
15.5
mΩ
23
8
–1.5
l
0.93
EN/UV Pin Hysteresis
0.98
mΩ
19
A
1.5
µA
1.03
40
–20
A
V
mV
EN/UV Pin Current
VEN/UV = 2V
20
nA
PG Upper Threshold Offset from VFB
VFB Rising
l
6
7.75
9.5
%
PG Lower Threshold Offset from VFB
VFB Falling
l
–9.5
–7.75
–6
%
80
nA
2000
Ω
1.5
2.9
V
V
V
PG Hysteresis
0.4
PG Leakage
VPG = 3.3V
PG Pull-Down Resistance
VPG = 0.1V
l
SYNC/MODE Threshold
SYNC/MODE DC and Clock Low Level Voltage
SYNC/MODE Clock High Level Voltage
SYNC/MODE DC High Level Voltage
l
l
l
Spread Spectrum Modulation Frequency Range
–80
RT = 38.3k
600
0.7
2.2
24
%
%
Rev. 0
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3
LT8638S
ELECTRICAL
CHARACTERISTICS
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER
CONDITIONS
MIN
Spread Spectrum Modulation Frequency
MAX
3
SS Source Current
l
1.3
SS Pull-Down Resistance
Fault Condition, SS = 0.1V
VIN to Disable Forced Continuous Mode
VIN Rising
35
PHMODE Thresholds
Between 180° and 120°
Between 120° and 90°
0.7
2.2
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT8638SE is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT8638SJ are guaranteed over the full –40°C to 150°C operating junction
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125˚C. The junction temperature (TJ, in °C) is calculated from the
ambient temperature (TA in °C) and power dissipation (PD, in Watts)
according to the formula:
TJ = TA + (PD • JA)
where JA (in °C/W) is the package thermal impedance.
4
TYP
2.0
kHz
2.7
200
37
UNITS
µA
Ω
39
V
1.5
2.9
V
V
Note 3: values determined per JEDEC 51-7, 51-12. See the Applications
Information section for information on improving the thermal resistance
and for actual temperature measurements of a demo board in typical
operating conditions.
Note 4: This IC includes overtemperature protection that is intended to
protect the device during overload conditions. Junction temperature will
exceed 150°C when overtemperature protection is active. Continuous
operation above the specified maximum operating junction temperature
will reduce lifetime.
Rev. 0
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LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
95
EFFICIENCY
EFFICIENCY (%)
90
2.4
80
POWER LOSS
75
1.6
L = XGL6060
400kHz, L = 2.2µH 0.8
1MHz, L = 1µH
2MHz, L = 0.68µH
0
3
4
5
6
7
8
9 10
LOAD CURRENT (A)
70
1
2
EFFICIENCY
POWER LOSS
75
1
3.6
90
3.0
EFFICIENCY
2.4
85
80
1.2
75
70
65
1.8
POWER LOSS
1
2
100
EFFICIENCY (%)
FC MODE
VIN = 12V
VIN = 24V
VIN = 36V 0.6
fSW = 400kHz, L = XGL6060, 2.2µH
0
3
4
5
6
7
8
9 10
LOAD CURRENT (A)
2
60
50
VIN = 12V
VIN = 24V
VIN = 36V
40
2
3
4
5
6
7
8
LOAD CURRENT (A)
10
0.1
10
0
1
10
100
1000
LOAD CURRENT (mA)
60
50
40
VIN = 12V
VIN = 24V
VIN = 36V
30
fSW = 400kHz
L = XGL6060, 2.2µH
Burst Mode OPERATION
0
0.1
10000
1
10
100
1000
LOAD CURRENT (mA)
8638S G05
10000
8638S G06
Reference Voltage
609
607
REFERENCE VOLTAGE (mV)
EFFICIENCY (%)
9
70
10
85
80
75
70
8638S G07
1
20
fSW = 400kHz
L = XGL6060, 2.2µH
Burst Mode OPERATION
20
90
2.2
0.6
fSW = 400kHz, L= XGL6060, 2.2µH
Light Load Efficiency at 3.3VOUT
70
95
1.2
VIN = 12V
VIN = 24V
VIN = 36V
100
80
98
1.8
8638S G03
Light Load Efficiency at 5VOUT
30
96
EFFICIENCY (%)
65
80
100
90
2.4
70
90
100
3.0
POWER LOSS
80
Burst Mode Operation Efficiency
vs Inductor Value
92
EFFICIENCY
85
90
Efficiency vs Frequency
94
3.6
75
400kHz, L = 2.2µH 0.7
1MHz, L = 1µH
2MHz, L = 0.68µH
0
3
4
5
6
7
8
9 10
LOAD CURRENT (A)
8638S G04
88 V = 12V
IN
VOUT = 3.3V
86 I
LOAD = 5A
L = XGL6060, 2.2µH
84
0.2
0.6
1
1.4
1.8
SWITCHING FREQUENCY (MHz)
90
4.2
FC MODE
8638S G02
4.2
95
3.5
1.4
L = XGL6060
70
POWER LOSS (W)
EFFICIENCY (%)
Efficiency at 3.3VOUT
95
2.1
80
8638S G01
100
4.2
2.8
85
65
100
POWER LOSS (W)
4.0
FC MODE
Efficiency at 5VOUT
4.9
POWER LOSS (W)
95
3.2
85
65
4.8
POWER LOSS (W)
90
100
EFFICIENCY (%)
FC MODE
5.6
EFFICIENCY (%)
100
12VIN to 3.3VOUT Efficiency
vs Frequency
EFFICIENCY (%)
12VIN to 5VOUT Efficiency
vs Frequency
65
0.5
VIN=12V
VIN=24V
1.4
VOUT = 5V
ILOAD = 50mA
L = XGL6060
2.3
3.2
4.1
INDUCTOR VALUE (µH)
5
8638S G08
605
603
601
599
597
595
593
591
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8638S G09
Rev. 0
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5
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
EN Pin Thresholds
Load Regulation
1.00
1.00
0.99
0.75
EN RISING
0.97
0.96
0.95
0.94
EN FALLING
0.93
0.92
–50 –25
0
VIN = 12V
VOUT = 5V
0.50
0.25
0
–0.25
–0.50
–1.00
0
2
4
6
LOAD CURRENT (A)
8638S G10
100
75
10
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
22
18
17
16
30
300
BOTTOM SWITCH
5
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
0.3
0.5
DUTY CYCLE
0.7
8638S G16
20
40
5% DC
19
17
–50 –25
0.9
0
25 50 75 100 125 150
TEMPERATURE (°C)
8638S G15
Minimum On-Time
35
VIN = 5V
VOUT SET TO REGULATE AT 5V
L = XGL6060, 1µH
250
200
150
100
FORCED CONTINOUS MODE
Burst Mode OPERATION
30
25
20
VOUT = 0.6V
ILOAD = 5A
fSW = 3MHz
50
0
35
18
MINIMUM ON-TIME (ns)
350
DROPOUT VOLTAGE (mV)
35
10
15
20
25
30
LOAD CURRENT (A)
21
Dropout Voltage
400
15
10
8638S G14
40
TOP SWITCH
5
Top FET Current Limit vs
Temperature
19
14
0.1
40
25
VOUT = 5V
ILOAD = 1A
FC MODE
8638S G12
23
Switch RDS(ON) vs Temperature
RDS(ON) (mΩ)
–0.20
20
8638S G13
6
–0.10
15
50
20
–0.05
CURRENT LIMIT (A)
INPUT CURRENT (µA)
CURRENT LIMIT (A)
VOUT = 5V
L = 1µH
Burst Mode OPERATION
IN REGULATION
125
25
0.00
Top FET Current Limit vs Duty Cycle
200
150
0.05
8638S G11
No-Load Supply Current
175
8
0.10
–0.15
FORCED CONTINUOUS MODE
Burst Mode OPERATION
–0.75
25 50 75 100 125 150
TEMPERATURE (°C)
0.15
CHANGE IN VOUT (%)
CHANGE IN VOUT (%)
EN THRESHOLD (V)
0.98
Line Regulation
0.20
0
1
2
3 4 5 6 7
LOAD CURRENT (A)
8
9
10
8638S G17
15
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
8638S G18
Rev. 0
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LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (MHz)
1.02
1.01
1.00
0.99
0.98
1000
0.6
800
600
400
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
0.2
8638S G19
VSS = 0.5V
2.1
2.0
1.9
1.8
200
100
0
–100
–200
–300
0
25 50 75 100 125 150
TEMPERATURE (°C)
–400
–200
8638S G22
VC = 1.25V
–100
0
100
FB PIN ERROR VOLTAGE (mV)
7.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
2.8
2.7
2.6
2.5
8638S G25
2.6
2.5
2.3
2.3
2.2
–50 –25
2.7
2.4
2.4
25 50 75 100 125 150
TEMPERATURE (°C)
25 50 75 100 125 150
TEMPERATURE (°C)
Minimum Input Voltage
2.8
0
0
8638S G24
2.9
–9.5
–50 –25
FB FALLING
6.0
–50 –25
200
2.9
–9.0
FB RISING
6.5
–6.5
–8.5
1.4
7.5
3.0
FB FALLING
1.2
PG Thresholds Above VREF
8.0
3.0
–8.0
0.6 0.8 1.0
SS VOLTAGE (V)
8.5
RT Programmed
Switching Frequency
FB RISING
0.4
9.0
–6.0
–7.5
0.2
8638S G23
PG Thresholds Below VREF
–7.0
0
8638S G21
9.5
300
2.2
1.6
–50 –25
0
1
Error Amp Output Current
1.7
PG THRESHOLD OFFSET FROM VREF (%)
0.8
400
VC PIN CURRENT (µA)
SS PIN CURRENT (µA)
2.3
0.4
0.6
LOAD CURRENT (A)
8638S G20
Soft-Start Current
2.4
0.4
0.2
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
200
0.97
0
0.8
FB VOLTAGE (V)
RT = 38.3k
1.03
0.96
–50 –25
Soft-Start Tracking
Burst Frequency
1200
PG THRESHOLD OFFSET FROM VREF (%)
1.04
fSW = 2MHz
0
25 50 75 100 125 150
TEMPERATURE (°C)
8638S G26
2.2
–50 –25
fSW = 2MHz
0
25 50 75 100 125 150
TEMPERATURE (°C)
8638S G27
Rev. 0
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7
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Bias Pin Current vs Switching
Frequency
Bias Pin Current vs Input Voltage
60
VBIAS = 5V
VOUT = 5V
23 f
SW = 1MHz
FC MODE
22
BIAS PIN CURRENT (mA)
BIAS PIN CURRENT (mA)
24
21
20
19
18
17
IOUT = 0A
IOUT = 1A
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
VIN = 12V
54 VOUT = 5V
VBIAS = 5V
48 FC MODE
42
36
30
24
18
12
IOUT = 0A
IOUT = 1A
6
0
40
0
0.5
1
1.5
2
2.5
SWITCHING FREQUENCY (MHz)
8638S G28
8638S G29
Case Temperature Rise
vs 12A Pulsed Load
Case
Case Temperature
Temperature Rise
Rise
130
VIN = 12V, fSW = 400kHz, L = 2.2µH
120
VIN = 24V, fSW = 400kHz, L = 2.2µH
110
VIN = 12V, fSW = 2MHz, L = 0.47µH
100
VIN = 24V, fSW = 2MHz, L = 0.47µH
90 DEMO BOARD ON STILL AIR
80 L = XEL6060, XEL6030
V
= 3.3V
70 OUT
60
50
40
30
20
10
0
0
2
4
6
8
LOAD CURRENT (A)
100
DC2929A DEMO BOARD
VIN = 12V
VOUT = 3.3V
fSW = 2MHz
STANDBY LOAD = 0.25A
1kHz PULSED LOAD = 12A
90
CASE TEMPERATURE RISE (°C)
CASE TEMPERATURE RISE (°C)
3
80
70
60
50
40
30
20
10
10
0
0
8638S G30
0.2
0.4
0.6
0.8
DUTY CYCLE OF 12A LOAD
1
8638S G31
Switch Rising Edge
CLKOUT Waveforms
VCLKOUT
2V/DIV
VSYNC
2V/DIV
VSW
2V/DIV
RISE TIME = 2.5ns
VSW
10V/DIV
VIN = 12V
ILOAD = 3A
8
2ns/DIV
8638S G32
500ns/DIV
8638S G33
fSW = 500kHz
PHMODE = 0V
SYNCHRONIZATION MODE
Rev. 0
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LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Waveforms, Burst Mode
Operation
Switching Waveforms, Full
Frequency Continuous Operation
IL
1A/DIV
IL
1A/DIV
VSW
5V/DIV
VSW
5V/DIV
500ns/DIV
8638S G34
8638S G35
FRONT PAGE APPLICATION
12VIN TO 5VOUT AT 1A
FC MODE
20µs/DIV
FRONT PAGE APPLICATION
12VIN to 5VOUT AT 10mA
Burst Mode OPERATION
Transient Response; 2.5A to 7.5A
Load Step
Transient Response; 100mA to
5.1A Load Step
ILOAD
5A/DIV
ILOAD
5A/DIV
FCM
VOUT
100mV/DIV
VOUT
100mV/DIV
Burst Mode OPERATION
20µs/DIV
FRONT PAGE APPLICATION
2.5A to 7.5A TRANSIENT
12VIN, 5VOUT, fSW = 1MHz
Cc = 220pF, RC = 12.1k
COUT = 47µF, CLEAD = 15pF
8638S G36
50µs/DIV
FRONT PAGE APPLICATION
100mA to 5.1A TRANSIENT
12VIN, 5VOUT, fSW = 1MHz
Cc = 220pF, RC = 12.1k
COUT = 47µF, CLEAD = 15pF
8638S G37
Rev. 0
For more information www.analog.com
9
LT8638S
TYPICAL PERFORMANCE CHARACTERISTICS
Conducted EMI Performance
(CISPR25 Conducted Emission Test with Class 5 Peak Limits)
60
VOLTAGE METHOD
50
AMPLITUDE (dBµV)
40
30
20
10
0
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
–10
–20
0
3
6
9
12
15
18
FREQUENCY (MHz)
DC2929A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 3.3V OUTPUT AT 10A, fSW = 2MHz
21
24
27
30
8638S G38
Radiated EMI Performance
(CISPR25 Radiated Emission Test with Class 5 Peak Limits)
50
VERTICAL POLARIZATION
PEAK DETECTOR
45
AMPLITUDE (dBµV/m)
40
35
30
25
20
15
10
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
5
0
0
100
200
300
400
500
600
FREQUENCY (MHz)
700
800
900
1000
8638S G39
50
HORIZONTAL POLARIZATION
PEAK DETECTOR
45
AMPLITUDE (dBµV/m)
40
35
30
25
20
15
10
CLASS 5 PEAK LIMIT
SPREAD SPECTRUM MODE
FIXED FREQUENCY MODE
5
0
0
100
200
300
400
500
600
FREQUENCY (MHz)
700
DC2929A DEMO BOARD
(WITH EMI FILTER INSTALLED)
14V INPUT TO 3.3V OUTPUT AT 10A, fSW = 2MHz
10
800
900
1000
8638S G40
Rev. 0
For more information www.analog.com
LT8638S
PIN FUNCTIONS
PHMODE (Pin 1): Pin determines the phase relationship
between the LT8638S’s internal clock and CLKOUT. Tie it to
GND for 2-phase operation, float the pin for 3-phase operation, or tie it to INTVCC for 4-phase operation. See Block
Diagram for internal pull-up and pull-down resistance.
BIAS (Pin 2): The internal regulator will draw current from
BIAS instead of VIN when BIAS is tied to a voltage higher
than 3.1V. For output voltages of 3.3V to 25V this pin
should be tied to VOUT. If this pin is tied to a supply other
than VOUT use a 1µF local bypass capacitor on this pin.
If no supply is available, tie to GND. However, especially
for high input or high frequency applications, BIAS should
be tied to output or an external supply of 3.3V or above.
INTVCC (Pin 3): Internal 3.4V Regulator Bypass Pin. The
internal power drivers and control circuits are powered
from this voltage. Do not load the INTVCC pin with external circuitry. INTVCC current will be supplied from BIAS
if BIAS > 3.1V, otherwise current will be drawn from VIN.
Voltage on INTVCC will vary between 2.8V and 3.4V when
BIAS is between 3.0V and 3.6V. Place a low ESR ceramic
capacitor of at least 1µF from this pin to ground close to
the IC.
BST (Pin 4): This pin is used to provide a drive voltage, higher than the input voltage, to the topside power
switch. Place a 0.1µF boost capacitor as close as possible
to the IC.
SW (Pins 5–8): The SW pins are the outputs of the internal power switches. Tie these pins together and connect
them to the inductor. This node should be kept small on
the PCB for good performance and low EMI.
GND (Pins 9–14, 20, Exposed Pad Pins 29–32): Ground.
Place the negative terminal of the input capacitor as close
to the GND pins as possible. The exposed pads should
be soldered to the PCB for good thermal performance. If
necessary due to manufacturing limitations Pins 29 to 32
may be left disconnected, however thermal performance
will be degraded.
VIN (Pins 15–18): The VIN pins supply current to the
LT8638S internal circuitry and to the internal topside
power switch. These pins must be tied together and be
locally bypassed with a capacitor of 4.7µF or more. Be
sure to place the positive terminal of the input capacitor
as close as possible to the VIN pins, and the negative
capacitor terminal as close as possible to the GND pins.
NC (Pins 19): No Connect. This pin is not connected to
internal circuitry and can be tied anywhere on the PCB,
typically ground.
EN/UV (Pin 21): The LT8638S is shut down when this
pin is low and active when this pin is high. The hysteretic threshold voltage is 0.98V going up and 0.94V going
down. Tie to VIN if the shutdown feature is not used. An
external resistor divider from VIN can be used to program
a VIN threshold below which the LT8638S will shut down.
RT (Pin 22): A resistor is tied between RT and ground to
set the switching frequency.
CLKOUT (Pin 23): Output Clock Signal for PolyPhase
Operation. In forced continuous mode, spread spectrum,
and synchronization modes, the CLKOUT pin provides a
50% duty cycle square wave of the switching frequency.
The phase of CLKOUT with respect to the LT8638S’s
internal clock is determined by the state of the PHMODE
pin. CLKOUT’s peak-to-peak amplitude is INTVCC to GND.
In Burst Mode operation, the CLKOUT pin will be low. Float
this pin if the CLKOUT function is not used.
SYNC/MODE (Pin 24): For the LT8638S, this pin programs
four different operating modes: 1) Burst Mode operation.
Tie this pin to ground for Burst Mode operation at low
output loads—this will result in low quiescent current.
2) Forced Continuous mode (FCM). This mode offers
fast transient response and full frequency operation
over a wide load range. Float this pin for FCM. When
floating, pin leakage currents should be 3V)
for forced continuous mode with spread spectrum
modulation. 4) Synchronization mode. Drive this pin with
a clock source to synchronize to an external frequency.
During synchronization the part will operate in forced
continuous mode.
Rev. 0
For more information www.analog.com
11
LT8638S
PIN FUNCTIONS
PG (Pin 25): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within ±7.75% of the final regulation voltage, and there
are no fault conditions. PG is also pulled low when EN/
UV is below 1V, INTVCC has fallen too low, VIN is too low,
or thermal shutdown. PG is valid when VIN is above 2.8V.
VC (Pin 26): The VC pin is the output of the internal error
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
SS (Pin 27): Output Tracking and Soft-Start Pin. This pin
allows user control of output voltage ramp rate during
start-up. A SS voltage below 1V forces the LT8638S to
regulate the FB pin to a function of the SS pin voltage. See
plot in the Typical Performance Characteristics section.
When SS is above 1V, the tracking function is disabled
12
and the internal reference resumes control of the error
amplifier. An internal 2µA pull-up current from INTVCC
on this pin allows a capacitor to program output voltage
slew rate. This pin is pulled to ground with an internal
200Ω MOSFET during shutdown and fault conditions; use
a series resistor if driving from a low impedance output.
This pin may be left floating if the soft-start feature is not
being used.
FB (Pin 28): The LT8638S regulates the FB pin to 0.6V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and VOUT.
Typically, this capacitor is 4.7pF to 47pF.
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground.
Rev. 0
For more information www.analog.com
LT8638S
BLOCK DIAGRAM
VIN
17–18
CIN2
10nF
VIN
15–16
VIN
CIN1
10nF
CIN3
R3
OPT
21
R4
OPT
26
0.98V
EN/UV
INTERNAL 0.6V REF
+
–
SHDN
CC
25
R2
PG
ERROR
AMP
R1
28
29
RT
3
0.1μF
CVCC
±7.75%
OSCILLATOR
200kHz TO 3MHz
BST
+
+
–
CSS
OPT
INTVCC
2
VC
VOUT
CPL
BIAS
3.4V
REG
SLOPE COMP
RC
CF
–
+
FB
SS
SHDN
THERMAL SHDN
INTVCC UVLO
VIN UVLO
SHDN
THERMAL SHDN
VIN UVLO
2µA
4
BURST
DETECT
CBST
M1
SWITCH
LOGIC
AND
ANTI-SHOOT
THROUGH
SW
5–8
L
VOUT
COUT
M2
GND
9–14, 20,
29–32
INTVCC
22
RT
24
23
INTVCC
INTVCC
SYNC/MODE
CLKOUT
PLL
60k
60k
600k
600k
PHMODE
1
8638S BD
Rev. 0
For more information www.analog.com
13
LT8638S
OPERATION
The LT8638S is a monolithic, constant frequency, current mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which the
top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the VFB pin with an internal
0.6V reference. When the load current increases it causes
a reduction in the feedback voltage relative to the reference
leading the error amplifier to raise the VC voltage until the
average inductor current matches the new load current.
When the top power switch turns off, the synchronous
power switch turns on until the next clock cycle begins
or in Burst Mode operation, inductor current falls to zero.
If overload conditions result in more than 15.5A flowing
through the bottom switch, the next clock cycle will be
delayed until switch current returns to a safe level.
The “S” in LT8638S refers to the second generation silent
switcher technology. This technology allows fast switching edges for high efficiency at high switching frequencies, while simultaneously achieving good EMI performance. This includes the integration of ceramic capacitors
into the package for VIN (see Block Diagram). These caps
keep all the fast AC current loops small, which improves
EMI performance.
If the EN/UV pin is low, the LT8638S is shut down and
draws approximately 6µA from the input. When the
EN/UV pin is above 0.98V, the switching regulator will
become active.
To optimize efficiency at light loads, the LT8638S operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 125µA (BIAS = 0). In a typical application, 90µA
(VIN = 12V, BIAS = 5VOUT) will be consumed from the
input supply when regulating with no load. The SYNC/
MODE pin is tied low to use Burst Mode operation and can
be floated to use forced continuous mode (FCM). If a clock
is applied to the SYNC/MODE pin, the part will synchronize
to an external clock frequency and operate in FCM.
14
The LT8638S can operate in forced continuous mode
(FCM) for fast transient response and full frequency operation over a wide load range. When in FCM the oscillator
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed.
The LT8638S can sink current from the output and return
this charge to the input in this mode, improving load step
transient response.
To improve EMI, the LT8638S can operate in spread spectrum mode. This feature varies the clock with a triangular frequency modulation of +24%. For example, if the
LT8638S’s frequency is programmed to switch at 2MHz,
spread spectrum mode will modulate the oscillator between
2MHz and approximately 2.5MHz. The SYNC/MODE pin
should be tied high to INTVCC (or >3V) to enable spread
spectrum modulation with forced continuous mode.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from VIN. The BIAS pin should be connected to
VOUT if the LT8638S output is programmed at 3.3V to 25V.
The VC pin allows the loop compensation of the switching regulator to be optimized based on the programmed
switching frequency, allowing for a fast transient response.
The VC and CLKOUT pins enable multiple LT8638S regulators to run with interleaving phase shift, reducing the
amount of required input and output capacitors. The
PHMODE pin selects the phasing of CLKOUT for different
multiphase applications.
Comparators monitoring the FB pin voltage will pull
the PG pin low if the output voltage varies more than
±7.75% (typical) from the set point, or if a fault condition
is present.
The oscillator reduces the LT8638S device’s operating
frequency when the voltage at the FB pin is low. This
frequency foldback helps to control the inductor current
when the output voltage is lower than the programmed
value which occurs during start-up or overcurrent conditions. When a clock is applied to the SYNC/MODE pin, the
SYNC/MODE pin is floated, or held DC high, the frequency
foldback is disabled and the switching frequency will slow
down only during overcurrent conditions.
Rev. 0
For more information www.analog.com
LT8638S
APPLICATIONS INFORMATION
Low EMI PCB Layout
The LT8638S is specifically designed to minimize EMI
emissions and also to maximize efficiency when switching at high frequencies. For optimal performance the
LT8638S should use multiple VIN bypass capacitors.
Two small 0.5), a
minimum inductance is required to avoid subharmonic
oscillation (Equation 9). See Application Note 19 for
more details.
LMIN =
VIN(2•DC−1)
5• fSW
(9)
where DC is the duty cycle ratio (VOUT/VIN) and fSW is the
switching frequency.
Rev. 0
For more information www.analog.com
19
LT8638S
APPLICATIONS INFORMATION
Input Capacitors
The VIN of the LT8638S should be bypassed with at least
three ceramic capacitors for best performance. Two small
ceramic capacitors of 3.1V
1µF OR GND
VC
RT
16.9k
0.33µH
VOUT
1.8V
10A
SW
22pF
100k
FB
49.9k
GND
47μF
x3
1210
X5R/X7R
8638S F16
fSW = 2MHz
L: XAL6030
Figure 16. 2MHz 1.8V, 10A Step-Down Converter
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Rev. 0
04/21
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