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TM
CM3406
1.5A, 210KHz Step-Down Converter
The Future of Analog IC Technology
TM
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
DESCRIPTION
The CM3406 is a monolithic step-down switch mode converter with a built-in internal power MOSFET. It achieves 1.5A continuous output current over a wide input supply range with excellent load and line regulation.
FEATURES
* * * * * * * * * * * * 1.5A Continuous Output Current 0.2 Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors Up to 95% Efficiency 20A Shutdown Mode Fixed 210KHz Frequency Thermal Shutdown Cycle-by-Cycle Over Current Protection Wide 4.75V to 22V Operating Input Range Output Adjustable from 1.23V to 18V Programmable Under Voltage Lockout Available in 8-Pin SO and PDIP Packages Distributed Power Systems Battery Chargers Pre-Regulator for Linear Regulators
Current mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle-by-cycle current limiting and thermal shutdown. The CM3406 requires a minimum number of readily available standard external components.
EVALUATION BOARD REFERENCE
Board Number EV3406DS-00A
TYPICAL APPLICATION
VIN 12V
A
CM3406 Rev. 1.6 10/29/2009
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
Dimensions
APPLICATIONS
* * *
2.3"X x 1.4"Y x 0.5"Z
"MPS" and "The Future of Analog IC Technology" are Trademarks of Monolithic Power Systems, Inc.
C4 10nF
Efficiency vs Load Current
3
2
ENABLE SHUTDOWN
8
EN
SW
EFFICIENCY (%)
4
VOUT 2.5V/1.5A
CM3406
OPEN NOT USED
1
NC
D1 B230A
FB
6
GND 5
COMP 7
C6 OPEN
C3 4.7nF
100 95 90 85 80 75 70 65 60 55 50 45 40
VOUT = 5V
VOUT = 3.3V
VOUT = 2.5V
VIN = 12V
1.00
0.25
0.50
0.75
1.25
1.50
LOAD CURRENT (A)
CM3406_TAC_S01
CM3406-EC01
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TM
CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
ORDERING INFORMATION
Part Number* CM3406DP CM3406DS Package PDIP8 SOIC8 Top Marking CM3406DS Temperature -40C to +85C
ABSOLUTE MAXIMUM RATINGS (1)
Supply Voltage (VIN)..................................... 24V Switch Voltage (VSW).................. -1V to VIN + 1V Bootstrap Voltage (VBS) ....................... VSW + 6V Feedback Voltage (VFB) .................-0.3V to +6V Enable/UVLO Voltage (VEN)...........-0.3V to +6V Comp Voltage (VCOMP) ...................-0.3V to +6V Continuous Power Dissipation (TA = +25C)(2) PDIP8 ...............................................1.3W SOIC8 ...............................................1.2W Junction Temperature ...............................150C Lead Temperature ....................................260C Storage Temperature.............. -65C to +150C
A
Recommended Operating Conditions
Input Voltage (VIN)..........................4.75V to 22V Operating Temperature............. -40C to +85C
CM3406 Rev. 1.6 10/29/2009
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PACKAGE REFERENCE
TOP VIEW
TOP VIEW
NC 1 2 3 4 8 7 6 5 EN
NC 1 2 3 4 8 7 6 5 EN
* FOR LEAD FREE, ADD SUFFIX -LF (EG. CM3406DP-LF) ** For Tape & Reel, add suffix -Z (eg. CM3406DS-Z) For Lead Free, add suffix -LF (eg. CM3406DS-LF-Z)
BS IN
COMP FB
BS IN
COMP FB
SW
GND
SW
GND
CM3406_PD01-PDIP8
CM3406_PD02-SOIC8
Thermal Resistance
(4)
PDIP8......................................95 ...... 55 ... C/W SOIC8 ....................................105 ..... 50 ... C/W
JA
JC
(3)
Notes: 1) Exceeding these ratings may damage the device. 2) The maximum allowable power dissipation is a function of the maximum junction temperature TJ(MAX), the junction-toambient thermal resistance JA, and the ambient temperature TA. The maximum allowable continuous power dissipation at any ambient temperature is calculated by PD(MAX)=(TJ(MAX)TA)/ JA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Internal thermal shutdown circuitry protects the device from permanent damage. 3) The device is not guaranteed to function outside of its operating conditions. 4) Measured on JESD51-7, 4-layer PCB.
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TM
CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
ELECTRICAL CHARACTERISTICS
VIN = 12V, TA = +25C, unless otherwise noted.
Parameter Feedback Voltage Upper Switch On Resistance Lower Switch On Resistance (5) Upper Switch Leakage Current Limit (4) Current Sense Transconductance Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance
(5)
Symbol Condition VFB 4.75V VIN 22V
Min 1.195
Typ 1.230 0.2 12
Max 1.265
Units V A A
Oscillator Frequency Short Circuit Frequency Maximum Duty Cycle Minimum Duty Cycle Enable Threshold Enable Pull Up Current Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis Supply Current (Shutdown) Thermal Shutdown Supply Current (Quiescent)
Note: 5) Guaranteed by design.
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
VEN = 0V, VSW = 0V 10 2.5 GCS GEA fS 1.85 400 700 AVEA IC = 10A 450 1000 VFB = 0V VFB = 1.0V VFB = 1.5V ICC > 100A VEN = 0V 210 130 90 0.7 1.0 2.37 1.0 1.2 2.50 210 20 0 1.3 2.62 VEN 0.4V 35 VEN 3.0 V, VFB =1.4V 0.9 1.1 160
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A/V
V/V
A/V KHz KHz % % V A V mV A mA C
A
CM3406 Rev. 1.6 10/29/2009
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CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
PIN FUNCTIONS
Pin # Name Description 1 2 No Connect. Open, not used. Bootstrap (C5). This capacitor is needed to drive the power switch's gate above the supply voltage. It is connected between SW and BS pins to form a floating supply across the power BS switch driver. The voltage across C5 is about 5V and is supplied by the internal +5V supply when the SW pin voltage is low. Supply Voltage. The CM3406 operates from a +4.75V unregulated input. C1 is needed to IN prevent large voltage spikes from appearing at the input. SW Switch. This connects the inductor to either IN through M1 or to GND through M2. Ground. This pin is the voltage reference for the regulated output voltage. For this reason care GND must be taken in its layout. This node should be placed outside of the D1 to C1 ground path to prevent switching current spikes from inducing voltage noise into the part. Feedback. An external resistor divider from the output to GND, tapped to the FB pin sets the output voltage. To prevent current limit run away during a short circuit fault condition the FB frequency foldback comparator lowers the oscillator frequency when the FB voltage is below 700mV. Compensation. This node is the output of the transconductance error amplifier and the input to COMP the current comparator. Frequency compensation is done at this node by connecting a series RC to ground. Enable/UVLO. There is about 7V internal zener connected between EN and GND as block diagram shows. The zener has 10mA maximum current rating. A voltage greater than 2.62V EN enables operation. Leave EN unconnected if unused. An Under Voltage Lockout (UVLO) function can be implemented by the addition of a resistor divider from VIN to GND. For complete low current shutdown it's the EN pin voltage needs to be less than 700mV. NC
A
CM3406 Rev. 1.6 10/29/2009
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
3 4 5 6 7 8
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TM
CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
BLOCK DIAGRAM
IN 2 INTERNAL REGULATORS 5V CURRENT SENSE AMPLIFIER SLOPE COMP CLK + --
OSCILLATOR
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
130/210KHz 5V 1 + + S Q M1 R Q 1.0V -SHUTDOWN COMPARATOR -CURRENT COMPARATOR 3 M2 7.0V -LOCKOUT COMPARATOR 1.8V 2.30V/ 2.50V + + -4 FREQUENCY FOLDBACK COMPARATOR -0.7V 1.23V FB + ERROR AMPLIFIER 5 6 COMP
BS
SW
EN 7
GND
CM3406_BD01
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio:
VFB = VOUT R2 R1 + R2
Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is:
VOUT = 1.23 x R1 + R2 R2
the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by:
L= V VOUT x 1 - OUT VIN f S x IL
A
R2 can be as high as 100k, but a typical value is 10k. Using that value, R1 is determined by:
Where fS is the switching frequency, IL is the peak-to-peak inductor ripple current and VIN is the input voltage.
R1 = 8.18 x ( VOUT - 1.23)
For example, for a 3.3V output voltage, R2 is 10k, and R1 is 17k. Inductor The inductor is required to supply constant current to the output load while being driven by
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CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by:
ILP = ILOAD VOUT V + x 1 - OUT 2 x fS x L VIN
prevent excessive voltage ripple at input. The input voltage ripple caused by capacitance can be estimated by:
VIN = ILOAD V V x OUT x 1 - OUT fS x C1 VIN VIN
Output Rectifier Diode The output rectifier diode supplies the current to the inductor when the high-side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current.
Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by:
I C1 = ILOAD x VOUT VOUT x1- VIN VIN

The worst-case condition occurs at VIN = 2VOUT, where:
IC1 = ILOAD 2
A
For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current.
The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1F, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to
CM3406 Rev. 1.6 10/29/2009
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
VOUT =
Where ILOAD is the load current.
Output Capacitor The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by:
VOUT V 1 Wh x 1 - OUT x RESR + fS x L VIN 8 x fS x C2
ere RESR is the equivalent series resistance (ESR) value of the output capacitor and C2 is the output capacitance value. In the case of ceramic capacitors, the output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by:
VOUT = VOUT
2
8 x fS
V x 1 - OUT VIN x L x C2
Where L is the inductor value.
In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to:
VOUT = VOUT V x 1 - OUT fS x L VIN x R ESR
The characteristics of the output capacitor also affect the stability of the regulation system. The CM3406 can be optimized for a wide range of capacitance and ESR values. Compensation Components The CM3406 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system.
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CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
The DC gain of the voltage feedback loop is:
A VDC = R LOAD x G CS x A VEA x VFB VOUT
The system has two poles of importance. One is due to the compensation capacitor (C3) and the output resistor of error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at:
fP1 GEA = 2 x C3 x A VEA
Where GEA is transconductance.
The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at:
f Z1 = 1 2 x C3 x R3
The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at:
fESR = 1 2 x C2 x R ESR
In this case, a third pole set by compensation capacitor (C6) and compensation resistor (R3) is used compensate the effect of the ESR zero on loop gain. This pole is located at:
A
The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is important.
CM3406 Rev. 1.6 10/29/2009
C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
R3 = 2 x C2 x f C VOUT x G EA x G CS VFB fP2 1 = 2 x C2 x R LOAD
Where RLOAD is the load resistor value, GCS is the current sense transconductance and AVEA is the error amplifier voltage gain.
Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system unstable. A good rule of thumb is to set the crossover frequency to below one-tenth of the switching frequency. To optimize the compensation components, the following procedure can be used: 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation:
Where fC is the desired crossover frequency, which is typically less than one tenth of the switching frequency.
the
error
amplifier
2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, to below one forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation:
C3 > 4 2 x R3 x f C
Where, R3 is the compensation resistor value. 3. Determine if the second compensation capacitor (C6) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid:
f 1 the the to the
If this is the case, then add the second compensation capacitor (C6) to set the pole fP3 at the location of the ESR zero. Determine the C6 value by the equation:
C6 = C2 x R ESR R3
fP 3 =
1 2 x C6 x R3
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CM3406 - 1.5A, 210KHz STEP-DOWN CONVERTER
MPS CONFIDENTIAL AND PROPRIETARY INFORMATION - ACCTON INTERNAL USE ONLY
PACKAGE INFORMATION
PDIP8
A
NOTICE: The information in this document is subject to change without notice. Please contact MPS for current specifications. Users should warrant and guarantee that third party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not assume any legal responsibility for any said applications.
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C C D TO M O NP N IN S O TC T EO D R NF IS N I TR A DE LN IB U T U SE IA TE L O N LY
SOIC8
PIN 1 IDENT. 0.229(5.820) 0.244(6.200) 0.150(3.810) 0.157(4.000) 0.0075(0.191) 0.0098(0.249)
SEE DETAIL "A"
0.013(0.330) 0.020(0.508)
0.011(0.280) x 45o 0.020(0.508)
0.050(1.270)BSC
0.189(4.800) 0.197(5.004)
0o-8o
0.053(1.350) 0.068(1.730)
0.049(1.250) 0.060(1.524)
0.016(0.410) 0.050(1.270)
DETAIL "A"
SEATING PLANE
0.001(0.030) 0.004(0.101)
NOTE: 1) Control dimension is in inches. Dimension in bracket is millimeters.
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