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MP1593
Monolithic Power Systems
General Description
The MP1593 is a step-down regulator with an internal Power MOSFET. It achieves 3A 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. Adjustable soft-start reduces the stress on the input source at turn-on. In shutdown mode the regulator draws 20A of supply current. The MP1593 requires a minimum number of readily available external components to complete a 3A step down DC to DC converter solution.
3A, 28V Step Down Converter
PRELIMINARY
Features
3A Output Current Programmable Soft-Start 100m Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors Up to 95% Efficiency 20A Shutdown Mode Fixed 385KHz Frequency Thermal Shutdown Cycle-by-Cycle Over Current Protection Wide 4.75 to 28V Operating Input Range Output Adjustable From 1.22 Under Voltage Lockout Available in 8 pin SOIC Package
Applications
Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators
Ordering Information
Part Number MP1593DN Package SOIC8 w/ Exposed Paddle Temperature -40 to + 85C
For Tape & Reel use suffix - Z (e.g. MP1593DN-Z)
Typical Application Circuit
INPUT 4.75to28V IN BS SW OUTPUT 2.5V,3A
EN S S
w
w
w
.d
ta a
he s
t4 e
u.
om c
GND
MP1593
COMP
FB
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MP1593
Monolithic Power Systems
Absolute Maximum Ratings (Note 1)
Supply Voltage VIN -0.3V to 30V -1V to VIN+0.3V Switch Voltage VSW Boost Voltage VBS VSW-0.3V to VSW+6V All Other Pins -0.3V to 6V Junction Temperature 150C Lead Temperature 260C Storage Temperature -65C to 150C
3A, 28V Step Down Converter
PRELIMINARY
Recommended Operating Conditions (Note 2)
Input Voltage VIN Ambient Operating Temperature 4.75V to 28V -40C to +85C
Package Thermal Characteristics
Thermal Resistance, JA (w/ Exposed Paddle) Thermal Resistance, JC (w/ Exposed Paddle)
(Note 3)
50C/W 10C/W
Electrical Characteristics (Unless otherwise specified VIN=12V, TA=25C)
Parameters
Shutdown Supply Current Supply current Feedback Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance High Side Switch On Resistance (Note 4) Low Side Switch On Resistance High Side Switch Leakage Current Current Limit Current Sense to COMP Transconductance Oscillation Frequency Short Circuit Oscillation Frequency Maximum Duty Cycle Minimum Duty Cycle EN Threshold Voltage Enable Pull Up Current Under Voltage Lockout Threshold Under Voltage Lockout Threshold Hysteresis Soft Start Period Thermal Shutdown
Condition
VEN = 0V VEN = 2.6V; VFB =1.4V 4.75V VIN 28V; VCOMP < 2V ICOMP = 10 A
Min
Typ
20 1.0
Max
30 1.2 1.250 1120 140 10 5.2 435 55 0 1.5 2.5 2.71
Units
A mA V V/V A/V m A A A/V KHz KHz % % V A V mV ms C
1.194 500
1.222 400 800 100 10 0 4.3 3.8 385 40 90 1.2 1.8 2.54 210 10 160
VEN=0V; VSW=0V 3.3 VFB = 0V VFB = 1.0V VFB = 1.5V VEN = 0V VIN Rising CSS = 0.1F 335 25 0.9 1.1 2.37
Note 1. Exceeding these ratings may damage the device. Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Measured on approximately 1" square of 1 oz. Copper with exposed pad area of 6.8mm2 (10.5mils2). Note 4: Guaranteed by design; not tested.
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MP1593
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Pin Description
3A, 28V Step Down Converter
PRELIMINARY
BS 1 IN 2 SW 3 GND 4
Exposed Pad on Backside. Connect to Pin 4.
8 SS 7 EN 6 COMP 5 FB
Table 1: Pin Designators #
1 2
Name Description
BS IN High-Side Gate Drive Boost Input. BS supplies the drive for the high-side n-channel MOSFET switch. Connect a 10nF or greater capacitor from SW to BS to power the high side switch. Power Input. IN supplies the power to the IC, as well as the step-down converter switches. Drive IN with a 4.75V to 28V power source. Bypass IN to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor section. Power Switching Output. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BS to power the high-side switch. Ground. (Note: Connect the exposed pad on backside to Pin 4). Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output voltage. The feedback threshold is 1.222V. See Setting the Output Voltage section. Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND to compensate the regulation control loop. In some cases, an additional capacitor from COMP to GND is required. See Compensation section. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator, drive EN low to turn it off. 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 its needs to be less than 0.7V. For automatic startup, leave EN unconnected. Soft Start Control Input. SS controls the soft start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.1F capacitor sets the soft-start period to 10ms. To disable the soft-start feature, leave SS unconnected.
3 4 5
SW GND FB
6
COMP
7
EN
8
SS
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MP1593
Monolithic Power Systems
3A, 28V Step Down Converter
PRELIMINARY
IN
2 Internal Regulators Slope Compensation Oscillator 40/385KHz S CLK R Shutdown Comparator 0.7V 1uA 1.8V Current Comparator 4 Q M2 Q 3 Current Sense Amplifier 5V M1 1
BS
SW
GND
EN
7
100K 2.285/2.495V Lockout Comparator
Frequency Foldback Comparator
0.7V 5
1.22V
Error Amplifier gm= 800uA/Volt 6
FB
COMP
8
SS
Figure 1: Functional Block
Functional Description
The MP1593 is a current-mode step-down regulator. It regulates input voltages from 4.75V to 28V down to an output voltage as low as 1.22V, and is able to supply up to 3A of load current. The MP1593 uses current-mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal error amplifier. The output current of the transconductance error amplifier is presented at COMP where a network compensates the regulation control system. The voltage at COMP is compared to the switch current measured internally to control the output voltage. The converter uses an internal n-channel MOSFET switch to step-down the input voltage to the regulated output voltage. Since the MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BS drives the gate. The capacitor is internally charged while SW is low. An internal 10 switch from SW to GND is used to insure that SW is pulled to GND when SW is low to fully charge the BS.capacitor.
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MP1593
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INPUT 4.75 to 28V C5 10nF IN BS SW EN OPEN NOT USED SS FB GND COMP C3 3.9nF R3 3.9K R1 10.5K R2 10K C2 22F/10V Ceramic L1 15H D1 OUTPUT 2.5V/3A
3A, 28V Step Down Converter
PRELIMINARY
C1 10F/35V
MP1593
C6 Open
Figure 2: MP1593 with Murata 22F, 10V Ceramic Output Capacitor
Application Information
Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB (see Figure 2). The voltage divider divides the output voltage down by the ratio:
VFB = VOUT * R2 / (R1 + R2)
Thus the output voltage is:
VOUT = 1.222 * (R1 + R2) / R2
R2 can be as high as 100K, but a typical value is 10K. Using that value, R1 is determined by:
R1 = 8.18 * (VOUT - 1.222) (K)
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 the switched input voltage. A larger value
MP1593 Rev 1.0_02/27/04
inductor results in less ripple current that in turn results in lower output ripple voltage. However, the larger value inductor has a larger physical size, higher series resistance, and/or lower saturation current. Choose an inductor that does not saturate under the worst-case load conditions. A good rule for determining the inductance is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum load current. Also, make sure that the peak inductor current (the load current plus half the peak-to-peak inductor ripple current) is below the 3.3A minimum current limit. The inductance value can be calculated by the equation:
L = (VOUT) * (VIN-VOUT) / (VIN * f * I)
Where VOUT is the output voltage, VIN is the input voltage, f is the switching frequency, and I is the peak-to-peak inductor ripple current. Table 2 lists a number of suitable inductors from various manufacturers.
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Table 2: Inductor Selection Guide
Vendor/ Model Sumida CR75 CDH74 CDRH5D28 CDRH5D28 CDRH6D28 CDRH104R Toko D53LC Type A D75C D104C D10FL Coilcraft DO3308 DO3316 Open Open Ferrite Ferrite 9.4 9.4 13.0 13.0 3.0 5.1 Shielded Shielded Shielded Open Ferrite Ferrite Ferrite Ferrite 5.0 7.6 10.0 9.7 5.0 7.6 10.0 11.5 3.0 5.1 4.3 4.0 Open Open Shielded Shielded Shielded Shielded Ferrite Ferrite Ferrite Ferrite Ferrite Ferrite 7.0 7.3 5.5 5.5 6.7 10.1 7.8 8.0 5.7 5.7 6.7 10.0 5.5 5.2 5.5 5.5 3.0 3.0 Core Type Core Material Package Dimensions (mm) W L H
3A, 28V Step Down Converter
PRELIMINARY be placed close to the MP1593. Output Capacitor The output capacitor is required to maintain the DC output voltage. Low ESR capacitors are preferred to keep the output voltage ripple low. The characteristics of the output capacitor also affect the stability of the regulation control system. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the ESR. The output voltage ripple is estimated to be:
VRIPPLE = 1.4 * VIN * (fLC/f) 2
Input Capacitor The input current to the step-down converter is discontinuous, and so a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. A low ESR capacitor is required to keep the noise at the IC to a minimum. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. The input capacitor value should be greater than 10F. The capacitor can be electrolytic, tantalum or ceramic. However since it absorbs the input switching current it requires an adequate ripple current rating. Its RMS current rating should be greater than approximately 1/2 of the DC load current. For insuring stable operation C1 should be placed as close to the IC as possible. Alternately a smaller high quality ceramic 0.1F capacitor may be placed closer to the IC and a larger capacitor placed further away. If using this technique, it is recommended that the larger capacitor be a tantalum or electrolytic type. All ceramic capacitors should
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Where VRIPPLE is the output ripple voltage, VIN is the input voltage, fLC is the resonant frequency of the LC filter, f is the switching frequency. In the case of tanatalum or lowESR electrolytic capacitors, the ESR dominates the impedance at the switching frequency, and so the output ripple is calculated as: VRIPPLE = I * RESR Where VRIPPLE is the output voltage ripple, I is the inductor ripple current, and RESR is the equivalent series resistance of the output capacitors. 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 rectifier. Tables 3 provides the Schottky rectifier part numbers based on the maximum input voltage and current rating.
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MP1593
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Table 3: Schottky Rectifier Selection Guide
VIN (Max) 15V 20V 2A Load Current Part Vendor Number 30BQ015 4 B220 1 SK23 6 SR22 6 20BQ030 4 B230 1 SK23 6 SR23 3, 6 SS23 2, 3 3A Load Current Part Vendor Number B320 SK33 SS32 B330 B340L MBRD330 SK33 SS33 1 1, 6 3 1 1 4, 5 1, 6 2, 3
3A, 28V Step Down Converter
PRELIMINARY
*
*
GCS is the current sense gain, (roughly the output current divided by the voltage at COMP), 3.8 A/V RLOAD is the load resistance (VOUT / IOUT where IOUT is the output load current)
26V
The system has 2 poles of importance, one is due to the compensation capacitor (C3), and the other is due to the output capacitor (C2). These are: fP1 = GEA / (2*AVEA*C3) Where P1 is the first pole, and GEA is the error amplifier transconductance (800A/V). and fP2 = 1 / (2*RLOAD *C2) The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). The zero is: fZ1 = 1 / (2*R3*C3) If large value capacitors with relatively high equivalent-series-resistance (ESR) are used, the zero due to the capacitance and ESR of the output capacitor can be compensated by a third pole set by R3 and C6. The pole is: fP3 = 1 / (2*R3*C6) The system crossover frequency (the frequency where the loop gain drops to 1, or 0dB) is important. A good rule of thumb is to set the crossover frequency to approximately 1/10 of the switching frequency. In this case, the switching frequency is 385KHz, so use a crossover frequency, fC, of 40KHz. Lower crossover frequencies result in slower response and worse transient load recovery. Higher crossover frequencies can result in instability.
Table 4 lists some rectifier manufacturers.
Table 4: Schottky Diode Manufacturers
# 1 2 3 4 5 6 Vendor Diodes, Inc. Fairchild Semiconductor General Semiconductor International Rectifier On Semiconductor Pan Jit International Web Site www.diodes.com www.fairchildsemi.com www.gensemi.com www.irf.com www.onsemi.com www.panjit.com.tw
Choose a rectifier who's maximum reverse voltage rating is greater than the maximum input voltage, and who's current rating is greater than the maximum load current. Compensation The system stability is controlled through the COMP pin. COMP 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 loop gain is: AVDC = (VFB / VOUT) * AVEA * GCS * RLOAD Where: * VFB is the feedback threshold voltage, 1.222V * VOUT is the desired output regulation voltage * AVEA is the transconductance error amplifier voltage gain, 400 V/V
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MP1593
Monolithic Power Systems
Table 5: Compensation Values for Typical Output Voltage/Capacitor Combinations
VOUT 2.5V 3.3V 5V 12V 2.5V 3.3V 5V 12V C2 22F Ceramic 22F Ceramic 22F Ceramic 22F Ceramic 560F/6.3V (30m ESR) 560F/6.3V (30m ESR) 470F/10V (30m ESR) 220F/25V (30m ESR) R3 3.9K 4.7K 7.5K 10K 10K 10K 10K 10K C3 3.9nF 3.3nF 2.2nF 2.7nF 15nF 18nF 27nF 27nF C6 None None None None None None None
3A, 28V Step Down Converter
PRELIMINARY fC 5.91x10-4*R3 Choose the compensation capacitor to set the zero to 1/4 of the crossover frequency. Determine the value by the following equation: C3 = 2 / (*R3*fC) 1.08x103 *(C2*VOUT/R32) Determine if the second compensation capacitor, C6 is required. It is required if the ESR zero of the output capacitor happens at less than four times the crossover frequency. Or: 8*C2*RESR*fC 1 Or: 1.49x10-4 *(R3*RESR/VOUT) 1
None
Choosing the Compensation Components The values of the compensation components given in Table 5 yield a stable control loop for the output voltage and capacitor given. To optimize the compensation components for conditions not listed in Table 5, use the following procedure: Choose the compensation resistor to set the desired crossover frequency (See Figure 3). Determine the value by the following equation: R3 = 2*C2*VOUT*fC / (GEA*GCS*VFB) Putting in the known constants and setting the crossover frequency to the desired 40kHz: R3 6.8x107*C2*VOUT The value of R3 is limited to 10K to prevent output overshoot at startup, so if the value calculated for R3 is greater than 10K, use 10K. In this case, the actual crossover frequency is less than the desired 40kHz, and is calculated by: fC = R3*GEA*GCS*VFB / (2*C2*VOUT) or
MP1593 Rev 1.0_02/27/04
where RESR is the equivalent series resistance of the output capacitor. If this is the case add the second compensation capacitor. Determine the value by the equation:
C6 = C2*RESR(max) / R3
Where RESR(MAX) is the maximum ESR of the output capacitor. Example: VOUT = 5V C2 = 22F Ceramic (ESR = 10m)
R3 (6.8x107 )(22x10-6)(5V) = 7480
Use the nearest standard value of 7.5K.
C3 (1.08x103)(22x10-6)(5)/(7.5x103)2 = 2.11nF
Use the nearest standard value of 2.2nF. 8* C2* RESR*fC = 0.002 which is less than 1, therefore no second compensation capacitor is required.
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MP1593
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INPUT 4.75 to 28V C5 10nF
3A, 28V Step Down Converter
PRELIMINARY
C1 10F/50V
IN
BS SW
L1 15H
EN OPEN NOT USED SS GND
MP1593
FB COMP C3 1.8nF R3 8.2K
OUTPUT 2.5V/3A
D1 C2 47F/6.3V Panasonic SP
R1 10.5K R2 10K
C6 Open
Figure 3: MP1593 with Panasonic 47F, 6.3V Special Polymer Output Capacitor
Figure 4: MP1593 Soft Start Plot: No soft-start cap Figure 5: MP1593 Start Up Plots: 0.01F soft-start cap (VIN=-5V, VOUT=2.5V, Load=1A resistive) (VIN=5V, VOUT =2.5V, Load=1A resistive)
Figure 6: MP1593 Start Up Plots: 0.1F soft-start cap (VIN=5V, VOUT =2.5V, Load=1A resistive)
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MP1593
Monolithic Power Systems
3A, 28V Step Down Converter
PRELIMINARY
100.0%
95.0%
90.0%
85.0%
Efficiency
80.0% VOUT = 5.0V 75.0% VOUT = 3.3V VOUT = 2.5V 70.0%
65.0%
60.0%
55.0%
50.0% 0 500 1000 1500 2000 2500 3000 3500
Load Current (mA)
Figure 7: MP1593 Efficiency vs Load (VIN = 10V)
100.0%
90.0%
Efficiency
80.0% VOUT = 5.0V VOUT = 3.3V VOUT = 2.5V 70.0%
60.0%
50.0% 0 500 1000 1500 2000 2500 3000 3500
Load Current (mA)
Figure 8: MP1593 Efficiency vs Load (VIN = 7V)
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MP1593
Monolithic Power Systems
Packaging
SOIC8N (Exposed Pad)
3A, 28V Step Down Converter
PRELIMINARY
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"
NOTE 2 0.013(0.330) 0.020(0.508) 0.050(1.270)BSC 0.011(0.280) x 45o 0.020(0.508)
0.189(4.800) 0.197(5.004) 0.053(1.350) 0.068(1.730) 0.049(1.250) 0.060(1.524)
0o-8o
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. 2) Exposed Pad; 2.55+/- 0.25mm x 3.38 +/- 0.44mm. Recommended Solder Board Area: 2.80mm x 3.82mm = 10.7mm2 (16.6mil2)
NOTICE: MPS believes the information in this document to be accurate and reliable. However, it is subject to change without notice. Please contact the factory for current specifications. No responsibility is assumed by MPS for its use or fit to any application, nor for infringement of patent or other rights of third parties
MP1593 Rev 1.0 02/27/04 (c) 2004 MPS, Inc. Monolithic Power Systems, Inc. 983 University Ave, Building A, Los Gatos, CA 95032 USA Tel: 408-357-6600 Fax: 408-357-6601 Web: www.monolithicpower.com 11

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