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Final Electrical Specifications
LT1612 Synchronous, Step-Down 800kHz PWM DC/DC Converter
April 2000
FEATURES
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DESCRIPTION
The LT(R)1612 is an 800kHz, synchronous step-down DC/ DC converter that operates from an input voltage as low as 2V. Internal 0.45 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. No-load quiescent current is 160A and shutdown current is less than 1A. 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.
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
APPLICATIONS
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Portable Devices Lithium-Ion Step-Down Converters 5V to 3.3V Conversion 2-Cell Alkaline Step-Down Converters
TYPICAL APPLICATION
0.1F VIN 2V VIN SHDN LT1612 C1 10F MODE VC 33k 330pF FB GND R2 232k 1% R1 215k 1% C2 68F 3.15V BOOST SW 100pF L1 10H VOUT 1.2V 500mA
Efficiency for LT1612 vs Linear Regulator VOUT = 1.2V
90 80 VIN = 2V VIN = 3V VIN = 2V (LINEAR)
EFFICIENCY (%)
70 60 50 40
VIN = 3V (LINEAR) 10 100 LOAD CURRENT (mA) 500
1612 * F01b
C1: TAIYO-YUDEN JMK325BJ106MN C2: PANASONIC EEFCDOF680R L1: SUMIDA CD43-100
1612 F01a
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Figure 1. 2V to 1.2V Converter
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.
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LT1612
ABSOLUTE MAXIMUM RATINGS
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
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER
TOP VIEW VC FB VIN GND 1 2 3 4 8 7 6 5 SHDN MODE BOOST SW
LT1612EMS8 MS8 PART MARKING LTMS
MS8 PACKAGE 8-LEAD PLASTIC MSOP
TJMAX = 125C, JA = 120C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IQ Quiescent Current
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C, VIN = VSHDN = 3V
CONDITIONS MODE = 5V MODE = 0V, Not Switching SHDN = 0V
q q q q
VFB
FB Voltage FB Line Regulation FB Pin Bias Current (Note 3)
q q
gm
Error Amplifier Transconductance Min Input Voltage Max Input Voltage 2
fOSC
Oscillator Frequency
q
fOSC Line Regulation Maximum Duty Cycle
q
Shutdown Threshold
Minimum Voltage for Active Maximum Voltage for Shutdown
2
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(Note 1)
BOOST Pin Voltage ....................................... VIN + 5.5V Junction Temperature ........................................... 125C Operating Temperature Range (Note 2) ... -40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
TOP VIEW VC 1 FB 2 VIN 3 GND 4 8 7 6 5 SHDN MODE BOOST SW
ORDER PART NUMBER LT1612ES8 S8 PART MARKING 1612
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 120C/ W
MIN
TYP 1 160
MAX 2 220 1 0.635 0.635 0.15 50
UNITS mA A A V V %/V nA mhos V V kHz kHz %/ V % %
0.605 0.60
0.62 0.62 0.02 7 250
5.5 700 550 85 80 2 0.2 800 1 90 900 1100
q q
V V
LT1612
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER SHDN Pin Current BOOST Pin Current Switch Current Limit (Note 4)
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C, VIN = VSHDN = 3V
CONDITIONS SHDN = 2V SHDN = 5V BOOST = VIN + 2V Duty Cycle = 0% MODE = OV MODE = 5V Burst Mode Operation Current Limit Switch Voltage Drop Rectifier Voltage Drop SW Pin Leakage MODE = 0V ISW = 500mA IRECT = 500mA VSW = 5V
q q q
MIN
TYP 10 30 4
MAX 15 45 900 900
UNITS A A mA mA mA mA
600 550
710 650 180 200 300
280 400 1
mV mV A
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 0C to 70C. Specifications over the -40C to 85C operating
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.
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. 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: VOUT = 0.62V (1 + R1/R2) VIN (Pin 3): Supply Pin. Bypass capacitor C1 must be right next to this pin. GND (Pin 4): Ground Pin. Connect directly to local ground plane. 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. 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. SHDN (Pin 8): Shutdown Pin. Pull this pin low for shutdown mode. Tie to a voltage between 2V and 5.5V for normal operation.
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LT1612
BLOCK DIAGRA
VIN 3
VC 1
FB 2
A1
0.62V SWITCH SWITCH DRIVER 5 SW
MODE
7 0.7V
SHDN
8
SHUTDOWN
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. 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
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RSENSE 0.08 BOOST DIODE 6 BOOST
+
V/I A2
-
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+ -
A3 FLIP-FLOP R ENABLE S Q RECTIFIER DRIVE RECTIFIER
- +
OSCILLATOR
SLOPE COMPENSATION
4 GND
1612 BD
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. 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.
LT1612
OPERATIO
Layout Hints
The LT1612 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layouts. 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. 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.
R1
R2
RC
VIN
C1
MULTIPLE VIAs
1612 F02
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
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 160A. This reduces power
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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 20mV/DIV AC COUPLED IL 200mA/DIV 5s/DIV
1612 F03
Figure 3. Output Voltage Ripple is 20mVP-P for the Circuit of Figure 1
CC 1 2 3 4 LT1612 8 7 6 5 C3 SHDN MODE
VOUT 200mV/DIV AC COUPLED IL 200mA/DIV
ILOAD 10mA TO 310mA
C2 L1
0.1ms/DIV
1612 F04
Figure 4. Transient Response for the Circuit of Figure 1 with the MODE Pin Tied to Ground or Floating
GND
VOUT
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
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LT1612
TYPICAL APPLICATIONS
Single Li-Ion to 2V Converter
0.1F VIN 2.7V TO 4.2V VIN SHDN LT1612 BURSTMODE MODE VC 30k 680pF FB GND 453k 1% 1M 1% 22F CERAMIC BOOST SW 20pF L1 10H VOUT 2V 500mA
Transient Response
85 80 VOUT 50mV/DIV
EFFICIENCY (%)
IL 200mA/DIV
LOAD STEP 125mA TO 300mA
VIN = 4V VOUT = 2V MODE = HIGH
100s/DIV
Burst Mode Operation
VOUT 20mV/DIV
IL 100mA/DIV VSHDN 5V/DIV
MODE = LOW
5s/DIV
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10F CERAMIC
C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100
1612 TA02
Li-Ion to 2V Converter Efficiency
VIN = 2.8V 75 VIN = 4.2V 70 65 VIN = 3.5V 60 55 50
1612 TA03
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10 100 LOAD CURRENT (mA)
1000
1612 TA04
Inrush Current at Start-Up
VOUT 2V/DIV
INRUSH CURRENT 200mA/DIV
1612 TA05
0.2ms/DIV
1612 TA06
LT1612
TYPICAL APPLICATIONS
5V to 2.5V Converter
0.1F VIN 5V VIN SHDN LT1612 BURSTMODE C1 10F CERAMIC MODE VC 30k 332k 680pF C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100 FB GND 1M C2 22F CERAMIC BOOST SW 20pF L1 10H VOUT 2.5V 500mA
EFFICIENCY (%)
PACKAGE DESCRIPTION
0.007 (0.18) 0.021 0.006 (0.53 0.015)
0 - 6 TYP SEATING PLANE 0.012 (0.30) 0.0256 REF (0.65) TYP 0.192 0.004 (4.88 0.10) 0.118 0.004** (3.00 0.102)
* 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
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP 0.053 - 0.069 (1.346 - 1.752) 8 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 7 6 5
0.016 - 0.050 (0.406 - 1.270)
*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
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5V to 2.5V Converter Efficiency
85 80 75 70 65 60 55 50 1
1612 TA07
10 100 LOAD CURRENT (mA)
1000
1612 TA08
Dimension in inches (millimeters) unless otherwise noted. MS8 Package 8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
0.040 0.006 (1.02 0.15) 0.034 0.004 (0.86 0.102) 0.118 0.004* (3.00 0.102) 8 76 5
0.006 0.004 (0.15 0.102)
1
23
4
MSOP (MS8) 1197
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
1
2
3
4
SO8 0996
7
LT1612
TYPICAL APPLICATIONS
2V to 0.9V Converter
0.1F VIN 2V VIN SHDN LT1612 C1 10F MODE VC 33k 330pF C1: TAIYO-YUDEN JMK325BJ106MN C2: PANASONIC EEFCDOF680R L1: SUMIDA CD43-100 FB GND R2 232k R1 105k C2 68F 3.15V BOOST SW 100pF
EFFICIENCY (%)
5V to 3.3V Converter
C3 0.1F VIN 5V VIN SHDN LT1612 C1 10F MODE VC R3 33k C4 680pF FB GND R2 232k 1% R1 1M 1% BOOST SW 20pF L1 10H VOUT 3.3V 500mA
EFFICIENCY (%)
C1: TAIYO-YUDEN LMK325BJ106MN C2: TAIYO-YUDEN LMK325BJ226MN L1: SUMIDA CD43-100
RELATED PARTS
PART NUMBER LTC 1474 LTC1701 LTC1707
(R)
DESCRIPTION Low IQ Step-Down Switching Regulator SOT-23 Step-Down Switching Regulator Monolithic Synchronous Step-Down Switching Regulator
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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Efficiency for LT1612 vs Linear Regulator. VOUT = 0.9V.
80 VOUT 0.9V 500mA 70 60 50 40 30 20 10 1 10 100 LOAD CURRENT (mA) 1000
1612 TA10
L1 10H
VIN = 2V
VIN = 3V VIN = 2V (LINEAR)
VIN = 3V (LINEAR)
1612 TA09
Efficiency
85 80 75 70 65 60
C2 22F
VIN = 5V VOUT = 3.3V
55 50
1612 TA01a
1
10 100 LOAD CURRENT (mA)
1000
1612 TA01b
COMMENTS 10A IQ, VIN from 3V to 18V, MSOP Package up to 300mA 500mA in SOT-23 Package, 1MHz Switching Frequency 500mA, VIN from 2.65V to 8.5V
1612i LT/TP 0400 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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