LT1612
Synchronous, Step-Down
800kHz PWM
DC/DC Converter
U
DESCRIPTIO
FEATURES
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■
■
■
■
■
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■
The LT®1612 is an 800kHz, synchronous step-down DC/
DC converter that operates from an input voltage as low
as 2V. Internal 0.45Ω switches deliver output currents up
to 500mA, and the 800kHz switching frequency allows the
use of small, low value ceramic input and output capacitors. Input voltage ranges from 5.5V down to 2V and
output voltage can be set as low as the 620mV reference.
The device features Burst ModeTM operation, keeping
efficiency high at light loads. Burst Mode operation can be
defeated by pulling the MODE pin high, enabling constant
switching throughout the load range for low noise.
Operates from Input Voltage As Low As 2V
Internal 0.7A Synchronous Switches
Uses Ceramic Input and Output Capacitors
620mV Reference Voltage
800kHz Fixed Frequency Switching
Programmable Burst Mode Operation
Low Quiescent Current: 160µA
8-Lead MSOP or SO Package
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APPLICATIO S
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■
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Portable Devices
Lithium-Ion Step-Down Converters
5V to 3.3V Conversion
2-Cell Alkaline Step-Down Converters
No-load quiescent current is 160µA and shutdown current
is less than 1µA. The device is available in 8-lead SO and
MSOP packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATION
Efficiency for LT1612 vs Linear Regulator
VOUT = 1.2V
0.1µF
L1
10µH
BOOST
VIN
SW
SHDN
LT1612
C1
10µF
MODE
VC
90
VOUT
1.2V
500mA
100pF
FB
GND
33.2k
330pF
R2
232k
1%
R1
215k
1%
C2
68µF
3.15V
EFFICIENCY (%)
VIN
2V
80
VIN = 2V
70
VIN = 3V
60
VIN = 2V (LINEAR)
50
40
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
Figure 1. 2V to 1.2V Converter
VIN = 3V (LINEAR)
1612 F01a
30
10
100
LOAD CURRENT (mA)
500
1612 • F01b
sn1612 1612fs
1
LT1612
U
W W
W
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Supply Voltage (VIN) ............................................... 5.5V
SW Pin Voltage ....................................................... 5.5V
FB Pin Voltage ............................................... VIN + 0.3V
VC Pin Voltage ........................................................... 2V
SHDN Pin Voltage ................................................... 5.5V
MODE Pin Voltage .................................................. 5.5V
BOOST Pin Voltage ....................................... VIN + 5.5V
Junction Temperature ........................................... 125°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
W
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PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
TOP VIEW
VC
FB
VIN
GND
1
2
3
4
8
7
6
5
SHDN
MODE
BOOST
SW
MS8 PACKAGE
8-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 200°C/ W
LT1612EMS8
MS8 PART MARKING
ORDER PART
NUMBER
TOP VIEW
VC 1
8
SHDN
FB 2
7
MODE
VIN 3
6
BOOST
GND 4
5
SW
LT1612ES8
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
LTMS
1612
TJMAX = 125°C, θJA = 120°C/ W
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V
SYMBOL PARAMETER
IQ
Quiescent Current
VFB
FB Voltage
CONDITIONS
MODE = 5V
MODE = 0V, Not Switching
SHDN = 0V
MIN
●
●
●
●
gm
MAX
UNITS
1
160
2
220
1
mA
µA
µA
0.62
0.62
0.635
0.635
V
V
FB Line Regulation
●
0.02
0.15
%/V
FB Pin Bias Current (Note 3)
●
7
50
nA
Error Amplifier Transconductance
2
5.5
Oscillator Frequency
●
700
550
fOSC Line Regulation
Maximum Duty Cycle
●
Shutdown Threshold
µmhos
250
Min Input Voltage
Max Input Voltage
fOSC
0.605
0.60
TYP
Minimum Voltage for Active
Maximum Voltage for Shutdown
●
●
85
80
800
900
1100
V
V
kHz
kHz
1
%/ V
90
%
%
2
0.2
V
V
sn1612 1612fs
2
LT1612
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V
SYMBOL PARAMETER
CONDITIONS
MIN
SHDN Pin Current
SHDN = 2V
SHDN = 5V
BOOST Pin Current
BOOST = VIN + 2V
Switch Current Limit (Note 4)
Duty Cycle = 0%
●
●
TYP
MAX
10
30
15
45
UNITS
µA
µA
4
mA
MODE = OV
600
710
900
mA
MODE = 5V
550
650
900
mA
Burst Mode Operation Current Limit
MODE = 0V
180
Switch Voltage Drop
ISW = 500mA
200
280
mV
Rectifier Voltage Drop
IRECT = 500mA
300
400
mV
SW Pin Leakage
VSW = 5V, VSHDN = 0V
1
µA
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1612E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
●
mA
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: Bias current flows out of the FB pin.
Note 4: Duty cycle affects current limit due to slope compensation.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage vs Temperature
Quiescent Current vs Temperature
0.64
FB Pin Bias Current
0
190
MODE = 0V
0.62
0.61
0.60
0.59
– 50
–25
0
25
50
TEMPERATURE (°C)
75
100
1612 • G01
–2
170
–4
FB PIN CURRENT (nA)
QUIESCENT CURRENT (µA)
FB VOLTAGE (V)
0.63
180
160
150
140
130
120
–6
–8
–10
–12
–14
–16
110
–18
100
– 50
–20
– 50
–25
0
25
50
TEMPERATURE (°C)
75
100
1612 • G02
– 25
25
50
0
TEMPERATURE (°C)
75
100
1612 • G03
sn1612 1612fs
3
LT1612
U W
TYPICAL PERFOR A CE CHARACTERISTICS
800
80
700
SWITCH CURRENT (mA)
60
50
40
30
20
OSCILLATOR FREQUENCY (kHz)
SHDN PIN CURRENT (µA)
1000
MODE = 0V
70
MODE = 5V
600
500
400
300
10
0
0
1
2
4
3
SHDN PIN VOLTAGE (V)
200
–50
5
–25
0
25
50
TEMPERATURE (°C)
1612 • G04
500
86
82
78
74
0
25
50
TEMPERATURE (°C)
700
600
500
400
–50
100
75
100
1612 • G07
–25
0
25
50
TEMPERATURE (°C)
100
Rectifier Voltage Drop
600
400
300
200
100
0
75
1612 • G06
RECTIFIER VOLTAGE DROP (mV)
90
SWITCH VOLTAGE DROP (mV)
600
–25
800
Switch Voltage Drop
94
70
–50
75
900
1612 • G05
Maximum Duty Cycle vs
Temperature
MAXIMUM DUTY CYCLE (%)
Oscillator Frequency vs
Temperature
Switch Current Limit vs
Temperature
SHDN Pin Bias Current
0
100
500
200
300
400
SWITCH CURRENT (mA)
600
1612 • G08
500
400
300
200
100
0
0
100
500
200
300
400
RECTIFIER CURRENT (mA)
600
1612 • G09
sn1612 1612fs
4
LT1612
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PIN FUNCTIONS
VC (Pin 1): Compensation Pin. This is the current sink/
source output of the error amplifier. By connecting an RC
network from this pin to ground, frequency response can
be tuned for a wide range of circuit configurations. The
voltage at this pin also sets the current limit, and if
grounded, the switch will remain in the OFF state.
SW (Pin 5): Switch Pin. Connect inductor and boost
capacitor here. Minimize trace area at this pin to keep EMI
down.
BOOST (Pin 6): This is the supply pin for the switch driver
and must be above VIN by 1.5V for proper switch operation. Connect the boost capacitor to this pin.
FB (Pin 2): Feedback Pin. This pin is the negative input to
the error amplifier. Connect the resistor divider tap to this
point which sets VOUT according to:
MODE (Pin 7): Burst Mode Operation Disable Pin. For
continuous switching operation (low noise), pull this pin
above 2V. For Burst Mode operation which gives better
light load efficiency, tie to ground. Output ripple voltage in
Burst Mode operation is typically 30mVP-P. See applications section for more information about this function.
VOUT = 0.62V (1 + R1/R2)
VIN (Pin 3): Supply Pin. Bypass capacitor C1 must be right
next to this pin.
SHDN (Pin 8): Shutdown Pin. Pull this pin low for shutdown mode. Tie to a voltage between 2V and 5.5V for
normal operation.
GND (Pin 4): Ground Pin. Connect directly to local ground
plane.
W
BLOCK DIAGRA
RSENSE
0.08Ω
BOOST DIODE
6 BOOST
VIN 3
VC 1
FB 2
–
+
+
A1
V/I
–
SLOPE
COMPENSATION
A2
0.62V
SWITCH
MODE
+
–
7
0.7V
SWITCH
DRIVER
5 SW
A3
FLIP-FLOP
R
ENABLE
SHDN
8
SHUTDOWN
Q
RECTIFIER
DRIVE
RECTIFIER
S
OSCILLATOR
4 GND
1612 BD
sn1612 1612fs
5
LT1612
U
OPERATIO
The LT1612 employs fixed frequency, current mode control. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes VOUT and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the VC pin either higher or lower.
Because switch current is proportional to the VC pin
voltage, this change causes the switch current to be
adjusted until VOUT is once again satisfied. Loop compensation is taken care of by an RC network from the VC pin
to ground.
Layout Hints
Inside this main loop is another that sets current limit on
a cycle-by-cycle basis. This loop utilizes current comparator A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connected to VIN. Current ramps up in the inductor linearly
at a rate of (VIN – VOUT)/L. Switch current is set by the VC
pin voltage and when the voltage across RSENSE trips the
current comparator, a reset pulse will be generated and the
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of VOUT/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
The ground terminal of input capacitor C1 should tie close
to Pin 4 of the LT1612. Doing this reduces dI/dt in the
ground copper which keeps high frequency spikes to a
minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid introducing dI/dt in the ground plane.
If the load is light and VC decreases below A3’s trip point,
the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.
The LT1612 switches current at high speed, mandating
careful attention to layout for proper performance. You
will not get advertised performance with careless layout.
Figure 2 shows recommended component placement for
a buck (step-down) converter. Follow this closely in your
PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (< 5mm) to the IC
package. As little as 10mm of wire or PC trace from CIN to
VIN will cause problems such as inability to regulate or
oscillation.
R1
R2
CC
RC
1
2
VIN
C1
LT1612
8
SHDN
7
MODE
3
6
4
5
C3
C2
L1
MULTIPLE
VIAs
1612 F02
GND
VOUT
Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and VC
Pin Is Kept Low. Lead Length to Battery Should Be Kept Short
sn1612 1612fs
6
LT1612
U
OPERATIO
Burst Mode Operation Defeat
To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160µA. This reduces power
consumption but also adds low frequency voltage ripple to
the output. Figure 3 shows switching waveforms for a 5V
to 3.3V converter running in Burst Mode operation. Output
voltage ripple is approximately 20mVP-P. If the MODE pin
is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.
VOUT
200mV/DIV
AC COUPLED
VOUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
IL
200mA/DIV
ILOAD
10mA TO 310mA
5µs/DIV
0.1ms/DIV
1612 F03
Figure 3. Output Voltage Ripple is 20mVP-P for
the Circuit of Figure 1
1612 F04
Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating
VOUT
200mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
10mA TO 300mA
0.1ms/DIV
1612 F05
Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present
sn1612 1612fs
7
LT1612
U
TYPICAL APPLICATIONS
Single Li-Ion to 2V Converter
Li-Ion to 2V Converter Efficiency
85
0.1µF
L1
10µH
BOOST
VIN
SW
SHDN
LT1612
MODE
VC
10µF
CERAMIC
30.1k
680pF
80
VOUT
2V
500mA
VIN = 2.8V
75
EFFICIENCY (%)
VIN
2.7V TO 4.2V
20pF
FB
GND
1M
1%
453k
1%
22µF
CERAMIC
VIN = 4.2V
70
65
VIN = 3.5V
60
55
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
50
1612 TA02
1
10
100
LOAD CURRENT (mA)
1000
1612 TA04
Transient Response
Burst Mode Operation
VOUT 20mV/DIV
VOUT 50mV/DIV
IL 200mA/DIV
IL 100mA/DIV
LOAD STEP
125mA TO 300mA
VIN = 4V
VOUT = 2V
MODE = HIGH
100µs/DIV
MODE = LOW
1612 TA03
5µs/DIV
1612 TA05
Inrush Current at Start-Up
VOUT 2V/DIV
INRUSH
CURRENT
200mA/DIV
VSHDN 5V/DIV
0.2ms/DIV
1612 TA06
sn1612 1612fs
8
LT1612
U
TYPICAL APPLICATIONS
5V to 2.5V Converter
5V to 2.5V Converter Efficiency
85
0.1µF
L1
10µH
BOOST
VIN
SW
SHDN
LT1612
MODE
VC
C1
10µF
CERAMIC
20pF
FB
GND
1M
30.1k
332k
680pF
80
VOUT
2.5V
500mA
C2
22µF
CERAMIC
75
EFFICIENCY (%)
VIN
5V
70
65
60
55
50
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1
1612 TA07
10
100
LOAD CURRENT (mA)
1000
1612 TA08
2V to 0.9V Converter
Efficiency for LT1612 vs Linear Regulator.
VOUT = 0.9V.
0.1µF
L1
10µH
BOOST
VIN
SW
SHDN
LT1612
C1
10µF
MODE
VC
33.2k
80
VOUT
0.9V
500mA
100pF
60
FB
GND
R2
232k
330pF
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
R1
105k
VIN = 2V
70
C2
68µF
3.15V
EFFICIENCY (%)
VIN
2V
VIN = 3V
50
VIN = 2V (LINEAR)
40
VIN = 3V (LINEAR)
30
20
1612 TA09
10
1
10
100
LOAD CURRENT (mA)
1000
1612 TA10
sn1612 1612fs
9
LT1612
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PACKAGE DESCRIPTION
Dimension in inches (millimeters) unless otherwise noted.
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.118 ± 0.004*
(3.00 ± 0.102)
8
7 6
5
0.118 ± 0.004**
(3.00 ± 0.102)
0.193 ± 0.006
(4.90 ± 0.15)
1
2 3
4
0.043
(1.10)
MAX
0.007
(0.18)
0.034
(0.86)
REF
0° – 6° TYP
0.021 ± 0.006
(0.53 ± 0.015)
SEATING
PLANE
0.009 – 0.015
(0.22 – 0.38)
0.0256
(0.65)
BSC
0.005 ± 0.002
(0.13 ± 0.05)
MSOP (MS8) 1100
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
sn1612 1612fs
10
LT1612
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PACKAGE DESCRIPTION
Dimension in inches (millimeters) unless otherwise noted.
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
8
7
6
5
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
SO8 1298
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
0°– 8° TYP
0.016 – 0.050
(0.406 – 1.270)
0.014 – 0.019
(0.355 – 0.483)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
2
3
4
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
sn1612 1612fs
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LT1612
U
TYPICAL APPLICATIO
5V to 3.3V Converter
Efficiency
C3
0.1µF
L1
10µH
BOOST
VIN
SW
SHDN
LT1612
MODE
C1
10µF
VC
VOUT
3.3V
500mA
20pF
FB
GND
R3
33.2k
C4
680pF
R2
232k
1%
R1
1M
1%
VIN = 5V
VOUT = 3.3V
80
75
EFFICIENCY (%)
VIN
5V
85
70
65
60
C2
22µF
55
50
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA01a
1
10
100
LOAD CURRENT (mA)
1000
1612 TA01b
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DESCRIPTION
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®
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sn1612 1612fs
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Linear Technology Corporation
LT/TP 1100 4K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1999
工商网监
湘ICP备2023018690号
