LT8642-1
18V, 10A Synchronous
Step-Down Silent Switcher
FEATURES
DESCRIPTION
Silent Switcher® Architecture
n Ultralow EMI Emissions
n Optional Spread Spectrum Modulation
n High Efficiency at High Frequency
n Up to 96% Efficiency at 1MHz, 12V to 3.3V
IN
OUT
n Up to 95% Efficiency at 2MHz, 12V to 3.3V
IN
OUT
n Wide Input Voltage Range: 2.8V to 18V
n 10A Output Current
n External Compensation: Fast Transient Response
and Current Sharing
n Low Quiescent Current Burst Mode® Operation
n 240µA I Regulating 12V to 1.2V
Q
IN
OUT
n Output Ripple: 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.
VIN (Pins 4, 5, 12, 13): The VIN pins supply current to
the LT8642-1 internal circuitry and to the internal topside
power switch. The LT8642-1 requires the use of multiple
VIN bypass capacitors. Two small 1µF capacitors should
be placed as close as possible to the LT8642-1, one
capacitor on each side of the device (CIN1, CIN2). A third
capacitor with a larger value, 4.7µF or higher, should be
placed near CIN1 or CIN2. See Applications Information
section for sample layout.
GND (Pins 6, 11, Exposed Pad Pin 21): 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 Pin 21 may
be left disconnected, however thermal performance will
be degraded.
BST (Pin 7): This pin is used to provide a drive voltage,
higher than the input voltage, to the topside power switch.
Place a 47nF boost capacitor as close as possible to the IC.
10
SW (Pins 8–10): 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.
EN/UV (Pin 14): The LT8642-1 is shut down when this
pin is low and active when this pin is high. The hysteretic threshold voltage is 0.99V going up and 0.95V 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 LT8642-1 will shut down.
SYNC/MODE (Pin 15): For the LT8642-1, 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.
CLKOUT (Pin 16): In forced continuous mode, spread
spectrum, and synchronization modes, the CLKOUT pin
will provide a ~200ns wide pulse at the switch frequency.
The low and high levels of the CLKOUT pin are ground and
INTVCC respectively, and the drive strength of the CLKOUT
pin is several hundred ohms. In Burst Mode operation,
the CLKOUT pin will be low. Float this pin if the CLKOUT
function is not used.
RT (Pin 17): A resistor is tied between RT and ground to
set the switching frequency.
SS (Pin 18): 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 LT8642-1 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
Rev. 0
For more information www.analog.com
LT8642-1
PIN FUNCTIONS
FB (Pin 20): The LT8642-1 regulates the FB pin to 0.597V.
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.
and the internal reference resumes control of the error
amplifier. An internal 1.9µA 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.
Corner Pins: These pins are for mechanical support only
and can be tied anywhere on the PCB, typically ground.
VC (Pin 19): 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.
BLOCK DIAGRAM
VIN
12, 13
VIN
VIN
4, 5
CIN3
CIN2
CIN1
R3
OPT
INTERNAL 0.597V REF
14
R4
OPT
19
RC
1
CC
0.99V
EN/UV
+
–
SHDN
R2
20
CSS
OPT
18
RT
17
FB
SS
3
±8%
OSCILLATOR
200kHz TO 3MHz
+
+
–
R1
INTVCC
2
CVCC
ERROR
AMP
VOUT
C1
BIAS
3.4V
REG
SLOPE COMP
VC
PG
–
+
BST
BURST
DETECT
SHDN
THERMAL SHDN
INTVCC UVLO
VIN UVLO
SHDN
THERMAL SHDN
VIN UVLO
1.9µA
SWITCH LOGIC
AND
ANTI-SHOOT
THROUGH
7
CBST
M1
SW
8–10
L
VOUT
COUT
M2
GND
6, 11, 21
RT
INTVCC
CLKOUT
16
60k
15
SYNC/MODE
600k
86421 BD
Rev. 0
For more information www.analog.com
11
LT8642-1
OPERATION
The LT8642-1 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
VC pin. The error amplifier servos the VC node by comparing the voltage on the VFB pin with an internal 0.597V
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
inductor current falls to zero. If overload conditions result
in more than 13.5A flowing through the bottom switch,
the next clock cycle will be delayed until switch current
returns to a safe level.
If the EN/UV pin is low, the LT8642-1 is shut down and
draws 0.75µA from the input. When the EN/UV pin is
above 0.99V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8642-1 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 230µA. In a typical application, 240µA will be
consumed from the input supply when regulating with no
load. Note that the current in the feedback resistor divider
appears to the output as load current. 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.
The LT8642-1 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
12
operates continuously and positive SW transitions are
aligned to the clock. Negative inductor current is allowed.
The LT8642-1 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 LT8642-1 can operate in spread
spectrum mode. This feature varies the clock with a triangular frequency modulation of +20%. For example, if
the LT8642-1’s frequency is programmed to switch at
2MHz, spread spectrum mode will modulate the oscillator
between 2MHz and 2.4MHz. 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 LT8642-1 output is programmed at 3.3V
or above.
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 pin also enables current sharing and a CLKOUT pin
enables synchronizing other regulators to the LT8642-1.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±8% (typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8642-1’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
LT8642-1
APPLICATIONS INFORMATION
Low EMI PCB Layout
For more detail and PCB design files refer to the Demo
Board guide for the LT8642-1.
The LT8642-1 is specifically designed to minimize EMI
emissions and also to maximize efficiency when switching
at high frequencies. For optimal performance the LT8642-1
requires the use of multiple VIN bypass capacitors.
Note that large, switched currents flow in the LT8642-1
VIN and GND pins and the input capacitors. The loops
formed by the input capacitors should be as small as
possible by placing the capacitors adjacent to the VIN and
GND pins. Capacitors with small case size such as 0402
or 0603 are optimal due to lowest parasitic inductance.
Two small 0.5), a minimum inductance is required to avoid subharmonic oscillation (see Equation 9). See Analog Devices Application
Note 19 for more details.
LMIN =
VIN (2 •DC−1)
5 • fSW
(9)
Rev. 0
For more information www.analog.com
17
LT8642-1
APPLICATIONS INFORMATION
where DC is the duty cycle ratio (VOUT/VIN) and fSW is the
switching frequency.
Input Capacitors
The VIN of the LT8642-1 should be bypassed with at least
three ceramic capacitors for best performance. Two small
ceramic capacitors of 3.1V
1µF OR GND
47pF
GND
41.2k
100k
FB
147k
220µF ×2
1206
X5R/X7R
86421 F09
fSW = 1MHz
L: XEL6030
Figure 9. 1V 10A Step-Down Converter with Soft-Start and Power Good
VIN
2.8V TO 18V
10µF
1μF
0603
VIN
EN/UV
GND
VIN
GND
BST
LT8642-1
1μF
0603
47nF 0.56µH
SW
PG
CLKOUT
SYNC/MODE
VC
SS
INTVCC
10.2k
680pF
10nF
RT
1μF
41.2k
BIAS
VOUT
1.2V
10A
100k
EXTERNAL
SOURCE >3.1V
1µF OR GND
47pF
100k
FB
GND
100k
220µF ×2
1206
X5R/X7R
86421 F10
fSW = 1MHz
L: XEL6060
Figure 10. 1.2V, 10A Step-Down Converter with Soft-Start and Power Good
VIN
2.8V TO 18V
FB1
BEAD
22µF
1210
22µF
1210
22µF
1210
EN/UV
10µF
0603
VIN
VIN
GND
GND
BST
LT8642-1
PINS NOT USED IN
THIS CIRCUIT:
CLKOUT, PG, SS
INTVCC
1µF
SYNC/MODE
11k
150pF
VC
RT
17.8k
10µF
0603
47nF
SW
BIAS
GND
1µF
0.3µH
EXTERNAL
SOURCE >3.1V
OR GND
VOUT
1.2V
10A
47pF
100K
FB
86421 F11
100k
220µF ×2
1206
X5R/X7R
fSW = 2MHz
L: XEL4030
FB1 BEAD: WE-MPSB 74279228600
Figure 11. Ultralow EMI 2MHz 1.2V, 10A Step-Down Converter with Spread Spectrum
Rev. 0
For more information www.analog.com
23
LT8642-1
TYPICAL APPLICATIONS
VIN
2.8V TO 18V
10µF
1μF
0603
EN/UV
VIN
VIN
GND
1μF
0603
GND
BST
PINS NOT USED IN
THIS CIRCUIT:
CLKOUT, PG, SS
47nF
LT8642-1
0.28µH
VOUT
1V
10A
SW
INTVCC
1µF
BIAS
SYNC/MODE
VC
RT
10.2k
330pF
17.8k
1µF
EXTERNAL
SOURCE >3.1V
OR GND
47pF
100k
FB
GND
220µF ×2
1206
X5R/X7R
147k
86421 F12
fSW = 2MHz
L: XEL6030
Figure 12. 2MHz 1V, 10A Step-Down Converter with Spread Spectrum
VIN
3.6V TO 18V
10µF
1μF
0603
EN/UV
VIN
GND
VIN
GND
BST
PINS NOT USED IN
THIS CIRCUIT:
CLKOUT, PG, SS
LT8642-1
SW
INTVCC
1µF
SYNC/MODE
VC
RT
6.98k
220pF
17.8k
1μF
0603
47nF
0.82µH
VOUT
3.3V
10A
BIAS
39pF
100k
FB
22.1k
GND
47µF ×2
1210
X5R/X7R
86421 F13
fSW = 2MHz
L: XEL6060
Figure 13. 2MHz 3.3V, 10A Step-Down Converter with Spread Spectrum
VIN
5.5V TO 18V
10µF
1μF
0603
EN/UV
VIN
GND
VIN
GND
1μF
0603
BST
PINS NOT USED IN
THIS CIRCUIT:
CLKOUT, SYNC/MODE,
PG, SS
LT8642-1
SW
47nF
1.5µH
VOUT
5V
10A
BIAS
30.1k
VC
INTVCC
RT
1nF
1μF
L: XEL6030
25.5k
47pF
100k
FB
13.7k
GND
100µF ×2
1210
X5R/X7R
86421 F14
fSW = 1.5MHz
Figure 14. 5V, 10A Step-Down Converter
24
Rev. 0
For more information www.analog.com
4.50 ±0.05
0.25 REF
0.70 REF
0.375
aaa Z
2×
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license For
is granted
implication or
otherwise under any patent or patent rights of Analog Devices.
more by
information
www.analog.com
3.50 ±0.05
1.65
2.65
SUGGESTED PCB LAYOUT
TOP VIEW
0.375
D
PACKAGE TOP VIEW
0.7500
5
0.2500
0.0000
0.2500
PIN 1
CORNER
0.7500
Y
aaa Z
1.2500
0.7500
0.2500
0.0000
0.2500
0.7500
1.2500
PACKAGE
OUTLINE
X
E
2×
// bbb Z
SYMBOL
A
A1
L
b
D
E
D1
E1
e
H1
H2
aaa
bbb
ccc
ddd
eee
fff
DETAIL B
H2
MOLD
CAP
ddd Z
Z
MIN
0.85
0.01
0.30
0.22
H1
NOM
0.95
0.02
0.40
0.25
3.00
4.00
1.65
2.65
0.50
0.25 REF
0.70 REF
DIMENSIONS
DETAIL C
0.10
0.10
0.10
0.10
0.15
0.08
MAX
1.05
0.03
0.50
0.28
20b
eee M Z X Y
fff M Z
Z
SUBSTRATE
DETAIL C
A1
20×
e/2
e
L
SUBSTRATE THK
MOLD CAP HT
NOTES
DETAIL A
DETAIL B
A
(Reference LTC DWG # 05-08-1689 Rev Ø)
e
11
b
16
10
D1
e
6
7
0.275
20
DETAIL A
PACKAGE BOTTOM VIEW
0.375
17
b
7
6
1
4
SEE NOTES
PIN 1 NOTCH
0.25 × 45°
TRAY PIN 1
BEVEL
LQFN 20 0519 REV Ø
PACKAGE IN TRAY LOADING ORIENTATION
LTXXXXXX
CORNER SUPPORT PAD CHAMFER IS OPTIONAL
COMPONENT
PIN 1
7
THE EXPOSED HEAT FEATURE MAY HAVE OPTIONAL CORNER RADII
DETAILS OF PIN 1 IDENTIFIER ARE OPTIONAL, BUT MUST BE
LOCATED WITHIN THE ZONE INDICATED. THE PIN 1 IDENTIFIER
MAY BE EITHER A MOLD OR MARKED FEATURE
5
6
METAL FEATURES UNDER THE SOLDER MASK OPENING NOT SHOWN
SO AS NOT TO OBSCURE THESE TERMINALS AND HEAT FEATURES
4
3. PRIMARY DATUM -Z- IS SEATING PLANE
2. ALL DIMENSIONS ARE IN MILLIMETERS
NOTES:
1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M-1994
0.375
E1
ccc M Z X Y
ccc M Z X Y
LQFN Package
20-Lead (3mm × 4mm × 0.95mm)
LT8642-1
PACKAGE DESCRIPTION
Rev. 0
25
LT8642-1
TYPICAL APPLICATIONS
3.3V, 10A Step-Down Converter
VIN
3.6V TO 18V
10µF
1μF
0603
EN/UV
VIN
VIN
GND
GND
1μF
0603
BST
PINS NOT USED IN
THIS CIRCUIT:
CLKOUT, SYNC/MODE,
PG, SS
LT8642-1
SW
47nF
1.5µH
VOUT
3.3V
10A
BIAS
15k
VC
INTVCC
RT
470pF
1μF
L: XEL6060
41.2k
18pF
100k
FB
22.1k
GND
47µF ×2
1210
X5R/X7R
86421 TA02
fSW = 1MHz
RELATED PARTS
PART
DESCRIPTION
COMMENTS
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LT8640S/
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LT8603
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ISD < 1µA, 6mm × 6mm QFN-40
LT8602
42V, Quad Output (2.5A + 1.5A + 1.5A + 1.5A) 95% Efficiency, 2.2MHz
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VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 25µA,
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LT8645S/
LT8646S
65V, 8A, Synchronous Step-Down Silent Switcher 2 with IQ = 2.5μA
VIN(MIN) = 3.4V, VIN(MAX) = 65V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, 6mm × 4mm LQFN-32
LT8640/
LT8640-1
42V, 5A, 96% Efficiency, 3MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5μA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, 3mm × 4mm QFN-18
LT8641
65V, 3.5A, 95% Efficiency, 3MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5μA
VIN(MIN) = 3V, VIN(MAX) = 65V, VOUT(MIN) = 0.81V, IQ = 2.5µA,
ISD < 1µA, 3mm × 4mm QFN-18
LT8609/
LT8609A
42V, 2A, 94% Efficiency, 2.2MHz Synchronous MicroPower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3V, VIN(MAX) = 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA,
ISD < 1µA, MSOP-10E
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower
Step-Down DC/DC Converter with IQ = 2.5µA
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E
LT8611
42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower StepDown DC/DC Converter with IQ = 2.5µA and Input/Output Current
Limit/Monitor
VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA,
ISD < 1µA, 3mm × 5mm QFN-24
26
Rev. 0
01/22
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