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LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
DESCRIPTION
KEY FEATURES Input range voltage 2.7 to 5.5V Maximum output current, 1.8A Internal or External Synchronization Output Voltage from VIN to 0.6V PFM or PWM mode under light load EMI support with force PWM mode Efficiency up to 95% No external Schottky diode Thermal shutdown Cycle by cycle switch current limit UVLO Short circuit protection 8 pin MLP
APPLICATIONS
The Microsemi LX1918 is a step down buck regulator with a synchronous rectifier. All MOSFET switches and compensation components are built in. The synchronous rectification eliminates the need of an external Schottky diode and maximizes efficiency (>95%) for a high step-down ratio and a low output voltage. The input supply voltage range is from 2.7V to 5.5V with a maximum output current of 1.8A. The LX1918 is a current mode PWM with optional PFM under light loads. The LX1918 can switch with a 1MHz free running internal oscillator or be synchronized with an external oscillator ranging from 500KHz to 1.25MHz; the high frequency allows for small size and low cost external components. The LX1918 output voltage is programmable with two resistors and can be tightly regulated down to 0.6V.
The LX1918 MODE feature can either maximize efficiency or reduce EMI depending on the needs of the application. High efficiency (MODE = GND) is obtained by allowing the LX1918 to enter a PFM mode with discontinuous inductor current. Low EMI (MODE=VCC) is obtained by only operating in PWM mode even at no load. The LX1918 features include soft start, cycle-by-cycle switch current limit, hiccup short circuit current limiting and thermal shut down. The LX1918 comes in small 8 pin MLP package (fully RoHS compliant).
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IMPORTANT: For the most current data, consult MICROSEMI's website: http://www.microsemi.com
Lower power embedded applications WLAN power system Portable design with single cell Lion or 3 cells NiMH/NiCd Low power portable storage application
PRODUCT HIGHLIGHT
VIN
PWR
VCC MODE
SW
VOUT @ 1.8 A
LX1918
ON OFF
E/S FB PGND SGND
LX1918 LX1918
PACKAGE ORDER INFO
TA (C) -40 to 85
LD
Plastic MLP 8-Pin 3x3mm LX1918CLD
RoHS Compliant / Pb-free
Note: Available in Tape & Reel. Append the letters "TR" to the part number.(i.e. LX1918CLD-TR)
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
ABSOLUTE MAXIMUM RATINGS
PACKAGE PIN OUT
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Supply Input Voltage (PWR, VCC)..................................................-0.3V to 6.5V Supply Input Transient Voltage (PWR, VCC).................................................10V Input Voltage (E/S, FB,MODE) ......................................... -0.3V to (VCC + 0.3V) Maximum Switch Voltage (SW)....................................... -0.3V to (VPWR + 0.3V) SW Peak Current (Internally Limited)............................................................3.6A Operating Ambient Temperature Range ........................................-40C to +85C Operating Junction Temperature Range.......................................-40C to +125C Maximum Operating Junction Temperature ................................................ 150C Storage Temperature Range...........................................................-65C to 150C Peak Package Solder Reflow Temp. (40 second max. exposure) ... 260C (+0, -5)
Note: Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of specified terminal.
1
FB SGND PGND SW
2 3 4
8 7 6 5
E/S MODE VCC PWR
LD PACKAGE
(Top View)
THERMAL DATA
1918 xxxx MSC
LD PACKAGE MARKINGS
"xxxx" Denote Date Code and Lot Identification
LD
Plastic MLP 3mm X 3mm 8-Pin 35 - 50C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA
RoHS / Pb-free 100% Matte Tin Lead Finish
Junction Temperature Calculation: TJ = TA + (PD x JA). The JA numbers are guidelines for the thermal performance of the device/pc-board system. All of the above assume no ambient airflow. The variation in overall thermal resistance, JA, is directly affected by heatsink area available on the PCB.
FUNCTIONAL PIN DESCRIPTION PIN 1 2 3 4 5 6 7 8 Name FB SGND PGND SW PWR VCC MODE E/S Description Feedback Pin - This pin is regulated to the internal reference voltage. Signal Ground - Low current signal ground reference for IC; also attaches to bottom pad. Connect to common ground reference. Ground - High current ground; connects to common ground reference. MOSFET Switch (Drain) Connects for inductor. Power Input - Connect a decoupled voltage source between 2.7V and 5.5V. Reference power supply for IC. Light Load Mode Select - Ground for Highest efficiency mode or pull high for Lowest EMI mode during light load operation. Enable and External Synchronization - Pull high to enable operation with internal oscillator. Apply an external oscillator signal to synchronize the PWM switch frequency. Apply logic low to cause IC to enter Shutdown mode.
PACKAGE DATA PACKAGE DATA
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
ELECTRICAL CHARACTERISTICS Unless otherwise specified, the following specifications apply over the operating ambient temperature2 -40C TA where otherwise noted and the following test conditions: VIN = 3.7V; VE/S = 3.7V; VMODE = GND Parameter
ENTIRE REGULATOR
85C3 except
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Symbol
Test Conditions
Min 2.7
LX1918 Typ 92 350 0.5 1 1 0.600 1.00 0.1
Max 6.0 500 1.0 0.612 50 1.25 1.0 0.4 1250
Units V % A % % V nA MHz A V kHz ns A V A C V
Input Voltage3 Efficiency1 Quiescent Current * Active * Shutdown Line Regulation1 Load Regulation1 Feedback Voltage FB Input Current Internal Oscillator frequency E/S Pin Bias Current E/S Logic High Input E/S Logic Low Input SYNC Capture Range SYNC Minimum Pulse Width MODE Bias Current MODE Logic High MODE Logic Low SW Pin Current Limit Thermal Shutdown Threshold1 Under Voltage Lockout
VPWR, VCC VOUT = 1.8V, IOUT = 200mA ICC VE/S = 3.6V; VFB = 1.0V; T = 25C VE/S = GND VIN = 2.7 to 5.5V IOUT = 10mA to 1.5A VFB = 0.5V to 0.7V VE/S = 3.6V; VMODE= 3.6V VE/S = GND VMODE= 3.6V High or Low; VMODE= 3.6V VE/S = GND: VMODE= VIN VMODE(HI) V MODE(LO) Peak Current VUVLO
VFB IFB FOSC VE/S(HI) VE/S(LO)
0.588 -50 0.75 1.6 500 100
1.0 0.9 x VIN 2.0 130 2.7 140
2.0 0.1 x VIN 3.6 150 2.7
Notes 1. 2. 3.
Guaranteed by design, but not tested. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient temperature. Functionality over the -40C to +85C operating temperature range is assured by design, characterization, and correlation.
ELECTRICALS ELECTRICALS
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
SIMPLIFIED BLOCK DIAGRAM
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PWR VCC
2.7A UVLO AND ENABLE LOGIC Soft Start Ramp and Cur Limit Clamp 0mA
Current Limit Slope Compensation VREF OR ADJ +
FB
-
gm Compensation
Error Amp
Cur Limit
Turn on Blanking R Q Thermal Overload
S
Q
Reverse Current Detect
E/S
CLOCK LOGIC
CCO
<1 MHZ LIMITER
SGND
MODE
Figure 1 - Simplified Block Diagram
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
+
-
+ Current Sense FET Drivers
+
SW
Synchronous FET Driver LOGIC
PGND
ELECTRICALS ELECTRICALS
Page 4
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
APPLICATION CIRCUITS
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VIN = 2.7 to 5.5V 4.7F
LX1918
PWR SW
L1 3.3H 47F 20K 10K R1 R2
40.2 0.1F
1.8V @ 1.5A
VCC E/S FB LX1918
MODE
ON OFF
PGND
SGND
Figure 2 - Typical Application: Highest Light Load Efficiency Mode During Light Load With Internal 1MHz PWM Frequency
VIN = 2.7 to 5.5V 4.7F
LX1918
PWR SW
L1 3.3H 47F 15k 10k R1 R2
40.2 0.1F
1.5V @ 1.5A
VCC E/S FB LX1918
MODE PGND SGND
500kHz
APPLICATIONS APPLICATIONS
Figure 3 - Typical Application: PWM Low EMI Mode Operation During Light Load With External 500kHz Synchronization
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
THEORY OF OPERATION
LX1918 is synchronous step down converter with input voltage range from 2.7V to 5.5V; and outputs ranging from 0.6V to 80% of VIN. The LX1918 can deliver a maximum output current up to 1.8A. The LX1918 has two selectable operating modes for light loads: high efficiency PFM (MODE=GND) or low noise forced PWM (MODE=VCC). Under heavy load, LX1918 always operates in PWM mode and switches at fixed internal 1MHz frequency or can be synchronized with an external oscillator clock with a frequency ranging from 500 KHz to 1.25MHz. Switching frequency The E/S pin has dual function. If E/S pin pulls HIGH, the 1MHz internal oscillator is enabled. If E/S pin is held LOW, the LX1918 will enter Shut Down mode. Designers can apply an external oscillator signal, 500 KHz to 1.25MHz, to E/S pin to synchronize the switching frequency to a system clock. When applying external oscillator signal, the MODE pin must pull up to VCC activating the forced PWM mode. High Efficiency Operation: PFM, PWM, and EMI force PWM modes The LX1918 has two selectable operating modes for light loads: high efficiency PFM (MODE=GND) or low noise forced PWM(MODE=VCC). Only one mode can be active at any time. Under light load operation, defined as IOUT <300mA, if the MODE pin = VCC, the continuous inductor current mode is selected. This forces fixed frequency PWM and synchronous rectification regardless of loading. The inductor ripple current and duty cycle remain the same, the synchronous rectifier allows the inductor current to go positive or negative preventing system from going into discontinuous conduction and preventing possible EMI associated with inductor ringing.
If MODE pin is LOW, discontinuous current is selected. The PWM frequency slows down as the load decreases and this reduces switching losses. In this mode, the NMOS switch turns off to prevent negative current flow and allows the inductor current to reduce to zero (which is known as discontinuous conduction mode). Comparing the efficiency of the two modes of operation under light loading, when IOUT = 25mA, LX1918 efficiency is 80% (MODE=GND) and 68% (MODE=VCC). Under heavy load (IOUT>500mA), LX1918 always operates in PWM mode, with an efficiency up to 92%. Protection: LX1918 provides thermal shutdown, UVLO, and current limit protection: Thermal shutdown: if the die temperature reaches 150C. UVLO: an under voltage lock out, LX1918 will shut down when VIN < 2.7V. Current limiting protection: o Switch current limit: The PMOS switch is limited to a peak current of 2.7A. The PMOS "switch on" cycle is terminated when the switch current limit is reached. o Soft start: Upon start up, the switch current is limited to a gradual rise of 0 to 2.7A in 100us. The synchronous rectifier remains off during soft start preventing reverse load current. o Hiccup mode: If the PMOS switch is current limited when it attempts to turns on (indicating an output short circuit) , the converter will enter a partial soft start cycle mode to prevent a runaway output current.
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A APPLICATIONS
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
EFFICIENCY 5V TO 3.3V
REGULATION 5V TO 3.3V
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100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 1 10 100 Output Current (mA)
- low emi - hi eff
0.4% 0.3% 0.2% 0.1% 0.0% -0.1% -0.2% -0.3% -0.4% -0.5% -0.6% 0 500 1000 Output Current (mA)
Load Regulation (%)
reg-low emi reg - hi eff
Efficiency
1000
10000
1500
EFFICIENCY 3.3V TO 1.8V
REGULATION 3.3V TO 1.8V
100.0% 90.0% 80.0% 70.0% Efficiency 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 1 10 100 Output Current (in mA) 1000 10000
0.6% 0.4% Regulation (%) 0.2% 0.0% -0.2% -0.4% -0.6% -0.8% 0 500 1000 1500 Output Current (in mA) reg-low emi reg - hi eff
- low emi - hi eff
LINE REGULATION (1.8V OUT)
OUTPUT CURRENT VS. DUTY CYCLE
Iout max DC Load vs Duty Cycle
1.0% Variation from ideal 0.5% 0.0% -0.5% -1.0% 2.7 3.2 3.7 4.2 Input Voltage 4.7 5.2
High n (100mA) Low EMI (100mA) High n (600mA) Low EMI (600mA)
1.9 1.8 Iout DC, Amps 1.7 1.6 1.5 1.4 1.3 0% 25% 50% Duty Cycle 75% 100%
C CHARTS
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
PACKAGE DIMENSIONS
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LD
8 Pin Plastic MLP Dual Exposed Pad
D
L
E2
E b
D2
top
bottom
A3 A
side
Dim A A1 A3 b D D2 e E E2 L
Note:
MILLIMETERS MIN MAX 0.80 1.00 0 0.05 0.20 REF 0.25 0.30 3.00 BSC 1.60 2.50 0.65 BSC 3.00 BSC 1.35 1.75 0.30 0.50
INCHES MIN MAX 0.0315 0.0394 0 0.0019 0.0079 REF 0.010 0.0118 0.1181 BSC 0.0630 0.0984 0.0260 BSC 0.1181 BSC 0.0531 0.0689 0.0071 0.0197
e
A1
1. Dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006") on any side. Lead dimension shall not include solder coverage.
MECHANICALS MECHANICALS
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 8
LX1918
TM (R)
1.8A Step-Down Synchronous Converter
PRODUCTION DATA SHEET
NOTES
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NOTES NOTES
PRODUCTION DATA - Information contained in this document is proprietary to Microsemi and is current as of publication date. This document may not be modified in any way without the express written consent of Microsemi. Product processing does not necessarily include testing of all parameters. Microsemi reserves the right to change the configuration and performance of the product and to discontinue product at any time.
Copyright (c) 2004 Rev. 1.1, 2005-07-05
Microsemi
Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 9

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