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MP1411
2A, 18V, 380KHz Step-Down Converter
The Future of Analog IC Technology
TM
DESCRIPTION
The MP1411 is a monolithic step-down switch mode converter with a built in internal power MOSFET. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. Current mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle-by-cycle current limiting and thermal shutdown. In shutdown mode the regulator draws 23A of supply current. Programmable soft-start minimizes the inrush supply current and the output overshoot at initial startup. The MP1411 requires a minimum number of readily available standard external components.
FEATURES
* * * * * * * * * * * * 2A Output Current 0.2 Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors Up to 95% Efficiency 23A Shutdown Mode Fixed 380KHz Frequency Thermal Shutdown Cycle-by-Cycle Over Current Protection Wide 4.75V to 18V Operating Input Range Output Adjustable from 0.92V to 16V Programmable Under Voltage Lockout Available in an MSOP10 with Exposed Pad Package Distributed Power Systems Battery Charger DSL Modems Pre-Regulator for Linear Regulators
APPLICATIONS
* * * *
"MPS" and "The Future of Analog IC Technology" are Trademarks of Monolithic Power Systems, Inc.
TYPICAL APPLICATION
INPUT 4.75V - 18V
4 2 BS SW
C5 10nF
IN EN
Efficiency vs Output Current
95 90
EFFICIENCY (%)
OPEN = AUTOMATIC STARTUP
9 10
5.0V 3.3V 2.5V
5 7
MP1411
SS GND 6 FB COMP 8
D1 B220A
VOUT 3.3V/2A
85 80 75 70 65 60 0 0.5 1.0 1.5
C6 OPEN
C3 3.9nF
2.0
2.5
MP1411_TAC_S01
OUTPUT CURRENT (A)
MP1411_EC01
MP1411 Rev. 1.2 12/5/2005
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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PACKAGE REFERENCE
ABSOLUTE MAXIMUM RATINGS (1)
Supply Voltage (VIN) .................................... 20V Switch Node Voltage (VSW).......................... 21V 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 SS Voltage (VSS)............................ -0.3V to +6V Junction Temperature.............................+150C Lead Temperature ..................................+260C Storage Temperature ..............-65C to +150C
TOP VIEW
NC BS NC IN SW 1 2 3 4 5 10 9 8 7 6 SS EN COMP FB GND
EXPOSED PAD CONNECT TO PIN 6
Recommended Operating Conditions
MP1411_PD01_MSOP10
(2)
Supply Voltage (VIN) ...................... 4.75V to 18V Operating Temperature.................-40C to +85C
Thermal Resistance
Part Number* MP1411DH * Package MSOP10 Temperature -40C to +85C
(3)
MSOP10 with Exposed Pad .. 105 ..... 19... C/W
Notes: 1) Exceeding these ratings may damage the device. 2) The device is not guaranteed to function outside of its operating conditions. 3) Measured on approximately 1" square of 1 oz copper.
JA
JC
For Tape & Reel, add suffix -Z (eg. MP1411DH-Z) For Lead Free, add suffix -LF (eg. MP1411DH-LF-Z)
ELECTRICAL CHARACTERISTICS
VIN = 12V, TA = +25C, unless otherwise noted.
Parameter Feedback Voltage Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Limit (4) Current Sense Transconductance Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance Oscillator Frequency Short Circuit Frequency Soft-Start Pin Equivalent Output Resistance Symbol Condition VFB RDS(ON)1 RDS(ON)2 VEN = 0V, VSW = 0V 2.8 GCS AVEA GEA fS VFB = 0V IC = 10A 550 4.75V VIN 18V Min 0.892 Typ 0.920 0.2 10 0 3.4 1.95 400 830 380 240 9 1150 Max 0.948 Units V A A A/V V/V A/V KHz KHz k
10
MP1411 Rev. 1.2 12/5/2005
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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ELECTRICAL CHARACTERISTICS (continued)
VIN = 12V, TA = +25C, unless otherwise noted.
Parameter Maximum Duty Cycle Minimum On Time EN Shutdown Threshold Enable Pull Up Current EN UVLO Threshold Rising EN UVLO Threshold Hysteresis Supply Current (Shutdown) Supply Current (Quiescent) Thermal Shutdown
Note: 4) Slope compensation changes current limit above 40% duty cycle.
Symbol Condition DMAX VFB = 0.8V tON ICC > 100A VEN = 0V VEN Rising VEN 0.4V VEN 3V
Min
0.7 2.37
Typ 90 100 1.0 1.0 2.50 210 23 1.1 160
Max
1.3 2.62 36 1.3
Units % ns V A V mV A mA C
PIN FUNCTIONS
Pin # 1 2 Name Description No Connect. Bootstrap. This capacitor (C5) is needed to drive the power switch's gate above the supply voltage. It is connected between the SW and BS pins to form a floating supply across the power switch driver. The voltage across C5 is about 5V and is supplied by the internal +5V supply when the SW pin voltage is low. NC No Connect. IN Supply Voltage. The MP1411 operates from a +4.75V to +18V unregulated input. C1 is needed to 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. GND Ground. This pin is the voltage reference for the regulated output voltage. For this reason care 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. FB Feedback. An external resistor divider from the output to GND, tapped to the FB pin, sets the output voltage. To prevent current limit runaway during a short circuit fault condition the frequency foldback comparator lowers the oscillator frequency when the FB voltage is below 400mV. COMP Compensation. This node is the output of the transconductance error amplifier and the input to the current comparator. Frequency compensation is done at this node by connecting a series R-C to ground. See the compensation section for exact details. EN Enable/UVLO. A voltage greater than 2.62V enables operation. Leave EN unconnected for automatic startup. 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 the EN pin voltage needs to be less than 700mV. SS Soft-Start. Connect SS to an external capacitor to program the soft-start. If unused, leave it open. NC BS
3 4 5 6
7
8
9
10
MP1411 Rev. 1.2 12/5/2005
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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OPERATION
The MP1411 is a current mode regulator. That is, the COMP pin voltage is proportional to the peak inductor current. At the beginning of a cycle: the upper transistor M1 is off; the lower transistor M2 is on (see Figure 1); the COMP pin voltage is higher than the current sense amplifier output; and the current comparator's output is low. The rising edge of the 380KHz CLK signal sets the RS Flip-Flop. Its output turns off M2 and turns on M1 thus connecting the SW pin and inductor to the input supply. The increasing inductor current is sensed and amplified by the Current Sense Amplifier. Ramp compensation is summed to Current Sense Amplifier output and compared to the Error Amplifier output by the Current Comparator. When the Current Sense Amplifier plus Slope Compensation signal exceeds the COMP pin voltage, the RS Flip-Flop is reset and the MP1411 reverts to its initial M1 off, M2 on state. If the Current Sense Amplifier plus Slope Compensation signal does not exceed the COMP voltage, then the falling edge of the CLK resets the Flip-Flop. The output of the Error Amplifier integrates the voltage difference between the feedback and the 0.92V bandgap reference. The polarity is such that an FB pin voltage lower than 0.92V increases the COMP pin voltage. Since the COMP pin voltage is proportional to the peak inductor current an increase in its voltage increases current delivered to the output. The lower 10 switch ensures that the bootstrap capacitor voltage is charged during light load conditions. External Schottky Diode D1 carries the inductor current when M1 is off.
IN 4 INTERNAL REGULATORS OSCILLATOR 240KHz/ 380KHz + SLOPE COMP CLK CURRENT SENSE AMPLIFIER + -5V
2 Q Q 5
BS
+
S R
0.7V EN 9
--
SHUTDOWN COMPARATOR LOCKOUT COMPARATOR
--
CURRENT COMPARATOR
SW
-2.50V/ 2.30V
+
+
--
1.8V
6
GND
FREQUENCY FOLDBACK COMPARATOR
--
0.4V 7
0.92V FB SS 10
+
ERROR AMPLIFIER 8 COMP
MP1411_BD01
Figure 1--Functional Block Diagram
MP1411 Rev. 1.2 12/5/2005
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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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
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
Where ILOAD is the load current. 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 absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by:
ICIN = ILOAD x VOUT VOUT x 1- VIN VIN

Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is:
VOUT = 0.92 x R1 + R2 R2
A typical value for R2 can be as high as 100k, but a typical value is 10k. Using that value, R1 is determined by:
R1 = 10.87 x ( VOUT - 0.92)
For example, for a 3.3V output voltage, R2 is 10k, and R1 is 25.8k. Inductor The inductor is required to supply constant current to the output load while being driven by 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= VOUT V x 1 - OUT fS x IL VIN
The worst-case condition occurs at VIN = 2VOUT, where:
ICIN = ILOAD 2
Where fS is the switching frequency, IL is the peak-to-peak inductor ripple current and VIN is the input voltage.
For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current.
MP1411 Rev. 1.2 12/5/2005
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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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 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 CIN VIN VIN
Compensation Components The MP1411 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. The DC gain of the voltage feedback loop is given by:
A VDC = R LOAD x G CS x A VEA x VFB VOUT
Where CIN is the input capacitance value. 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 = VOUT V x 1 - OUT fS x L VIN 1 x R ESR + 8 x fS x C O
Where RLOAD is the load resistor value, GCS is the current sense transconductance and AVEA is the error amplifier voltage gain. 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 = fP 2 = GEA 2 x C3 x A VEA 1 2 x C O x R LOAD
Where L is the inductor value, RESR is the equivalent series resistance (ESR) value of the output capacitor and CO is the output capacitance value. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by:
VOUT = VOUT 8 x fS
2
Where GEA is transconductance.
the
error
amplifier
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
V x 1 - OUT VIN x L x CO

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 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 C O x R ESR
The characteristics of the output capacitor also affect the stability of the regulation system. The MP1411 can be optimized for a wide range of capacitance and ESR values.
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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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:
f P3 = 1 2 x C6 x R3
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 = C O x R ESR R3
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. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause the system to become 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:
R3 = 2 x C O x f C VOUT x G EA x G CS VFB
External Bootstrap Diode For applications with large duty cycles, it is recommended that an external boost diode be connected to a fixed 5V. This helps improve the efficiency of the MP1411 regulator and also avoids the problems caused by the decrease of BS voltage with large duty cycles. The fixed 5V can be pulled from the input of the system the output generated by the power supply. The boost diode can be a low cost one such as IN4148 or BAT54.
5V BOOTSTRAP DIODE
BS 2
MP1411
SW 5
10nF
MP1411_F02
Figure 2--External Bootstrap Diode Power Dissipation and Temperature Rise The power dissipation of the MP1411 is mostly from the conduction loss of the internal main switch. This power loss is estimated to be:
PLOSS VOUT 2 x IOUT x 0.18 x 1.3 VIN
Where fC is the desired crossover frequency, which is typically less than one tenth of the switching frequency. 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:
2 C3 > x R3 x f C
Where 1.3 is a temperature coefficient factor that reflects the increase in the RDS(ON) resistance at elevated temperatures. For example: for VIN = 12V, VOUT = 3.3V and IOUT = 2A:
PLOSS 3 .3 V x (2A ) 2 x 0.18 x 1.3 = 0.26 W 12V
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 MP1411 Rev. 1.2 12/5/2005
Because the thermal resistance JA is 105C/W, the resulting rise in temperature between junction and ambient is approximately 27C. Therefore, caution must be exercised when using the MP1411 in applications with high duty cycles.
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MP1411 - 2A, 18V, 380KHz STEP-DOWN CONVERTER
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PACKAGE INFORMATION
MSOP10 (WITH EXPOSED PAD)
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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