The MP1583 is a step-down regulator with abuilt-in internal Power MOSFET. It achieves 3A of 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. An adjustable soft-startreduces thestress on the input source at startup. In shutdown mode the regulator draws 20A of supply current.
The MP1583 requires a minimum number of external components, providing a compactsolution.
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 4.75V to 23V OperatingInput Range
•Output Adjustable from 1.22V to 21V
• Under-Voltage Lockout
APPLICATIONS
• Distributed Power Systems
• Battery Chargers
•Pre-Regulator for Linear Regulators
TYPICAL APPLICATION
INPUT
4.75V to 23V
OPEN =
AUTOMATIC
STARTUP
CERAMIC
10μF
2
7
EN
8
10nF
MP1583
SS
GNDCOMP
BSIN
1
SW
FB
64
B330A
5.6nF
3.9kΩ
3
5
10nF
15μH
10.5kΩ
10kΩ
“MPS” and “TheFuture ofAnalog ICTechnology” areRegisteredTrademarks ofMonolithic Power Systems, Inc.
2)The maximum allowable power dissipation is a function of themaximum junction temperature Tambient thermal resistance
. The maximum allowable continuous power dissipation at
T
A
any ambient temperature is calculated by PT
)/JA. Exceeding the maximum allowable power dissipation
A
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.
4GNDGround. (Note: For theSOIC8E package, connect theexposed pad on backside toPin4).
5 FB
6 COMP
High-Side Gate Drive Bootstrap Input. BS supplies the drive forthe high-side N-Channel MOSFET switch. Connect a4.7nF or greater capacitorfrom SWtoBS topower the high-side switch.
Power Input. INsupplies the power to the IC. Drive INwith a4.75V to23V powersource. Bypass
IN to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor
Power Switching Output. SW is the switching node that suppliespower to the output. Connect the output LC filter from SW to the output load. Note that acapacitor isrequired from SW to BS topower the high-side switch.
FeedbackInput. FB senses the outputvoltage and regulates it. DriveFB with a resistivevoltage
divider from the output voltage.The feedback thresholdis1.222V. SeeSetting the Output Voltage
Compensation Node. COMP isusedto compensatethe regulation control loop. Connect a series
RC network from COMPto GND to compensatethe regulation controlloop. See Compensation
Enable/UVLO. A voltage greater than 2.71V enables operation. For complete low current shutdown the EN pin voltage needs to be at less than 900mV. When the
7 EN
voltage on EN exceeds 1.2V, the internal regulator will be enabled and the soft-start capacitor will begin to charge. The MP1583 will start switching after the EN pin voltage reaches 2.71V. There is 7V zener connected between EN and GND. If EN is driven by external signal, the voltage should never exceed 7V.
8 SS
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from SS to GND toset the soft-start period. To disable the soft-start feature,leave SS unconnected.
OPERATION
The MP1583 is acurrent-modestep-downregulator. It regulates input voltages from 4.75V to23V down to an output voltage aslow as 1.222V, and is able to supply up to 3A of load current.
The MP1583 usescurrent-mode control toregulate the outputvoltage. The output voltage ismeasured at FB througharesistive voltage dividerand amplified through the internalerroramplifier.The output current of the transconductance error amplifier is presented at COMP where aRC network compensates theregulation control system.
The voltage at COMP is compared to theinternally measured switch current to control theoutput voltage.
The converter usesan internal N-Channel MOSFET switch to step-down the input voltagetothe regulated output voltage. Sincethe MOSFETrequires a gate voltage greater than the input voltage, aboostcapacitor connected between SW and BS drivesthe gate. The capacitor isinternally charged when SW is low.
An internal10switch from SW to GND is usedto insure that SWis pulled to GND whenSW is low in order to fully charge the BS capacitor.
The output voltage is set using a resistive voltage divider from the output voltage to the FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio:
2
R
VV
=
OUTFB
Where V
is the feedback voltage and V
FB
the output voltage.
Thus the output voltage is:
VV
22.1
OUT
×=
A typical value for R2 can be as high as 100k, but a typical value is 10k. Using that value, R1is determined by:
OUT
For example, for a 3.3V output voltage, R2 is10k, 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 valueinductor will result in less ripple current and lower output ripple voltage. However, larger value inductors have a larger physical size,higher series resistance, and/or lower saturation current. A good rule fordeterminingthe inductance to useis to allow the inductor peak-to-peak ripple current 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:
21
RR
+
is
OUT
RR
21
+
R
2
))(22.1(18.81Ω−×=kVVR
Choose an inductor that will not saturate under the maximum inductor peak current.
The peak inductor current can be calculated by:
⎞
V
OUT
⎟⎟
V
IN
⎠
Where I
II
LOADLP
is the load current.
LOAD
V
+=
OUT
S
⎛⎜
1
−×
⎜
Lf2
××
⎝
Table 1 lists a number of suitable inductors from various manufacturers. The choice ofwhich inductor to use mainly depends on the price vs. size requirements and any EMIrequirements.
The output rectifier diode supplies the current tothe inductor when the high-side switch is off.Use a Schottky diode to reduce losses due tothe diode forward voltage and recovery times.
Choose a diode whose maximum reverse voltage rating is greater than the maximum inputvoltage, and whose current rating is greater than themaximum load current. Table 2lists exampleSchottky diodes and manufacturers.
Table 2—Diode Selection Guide
Diode
SK33 30V, 3A Diodes Inc.
SK34 40V, 3A Diodes Inc.
B330 30V, 3A Diodes Inc.
B340 40V, 3A Diodes Inc.
MBRS330 30V, 3A On Semiconductor
MBRS340 40V, 3A On Semiconductor
oltage/Current
Rating
Manufacture
Input Capacitor
The input current to the step-down converter is discontinuous, therefore a capacitor is requiredto supply the AC current to the step-down converter while maintaining the DC inputvoltage. Use low ESR capacitors for the bestperformance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitorswill 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:
⎛
V
⎜
OUT
×
−×=
II
1
LOADC
1
⎜⎜
V
IN
⎝
⎞
V
⎟
OUT
⎟⎟
V
IN
⎠
The input capacitor canbe electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramiccapacitor (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 providesufficient charge to prevent excessive voltageripple at the input. The input voltage ripple caused by capacitance can be estimated by:
⎞
V
OUT
⎟
⎟
V
IN
⎠
V
OUT
IN
⎛⎜
1
−××
⎜⎝
I
V
IN
LOAD
=Δ
×
S
V
Cf
1
Where C1 is the input capacitance value.
Output Capacitor
The output capacitor is required to maintain theDC output voltage. Ceramic, tantalum or low ESR electrolytic capacitors are recommended.Low ESR capacitors are preferred so as to keep the output voltage ripple low. The output voltage ripple can be estimated by:
⎛
V
V
OUT
OUT
=Δ
S
⎛⎜
1
−×
⎜
Lf
×
⎝
⎞
V
OUT
V
⎜
⎟
⎟
IN
⎠
+×
R
ESR
⎜⎝
1
××
Cf
28
S
Where L is the inductor value, C2 is the outputcapacitance value and R
is the equivalent
ESR
series resistance (ESR) value of the output capacitor.
In the case of ceramic capacitors, theimpedance at the switching frequency is dominated by the capacitance, which is the main cause for the output voltage ripple. Forsimplification, the output voltage ripple can be estimated by:
⎞⎟
⎟⎠
The worst-case condition occurs at Vwhere:
I
I=
LOAD
1
C
2
For simplification, choose an input capacitor whose RMS current rating is greater than half of
IN
= 2V
OUT
,
ΔV
OUT
=
In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at theswitching frequency. For simplification, theoutput ripple can be approximated to:
The MP1583 can be optimized for a wide range of capacitance and ESR values.
Compensation Components
The MP1583 employs current mode control foreasy compensation and fast transient response.The system stability and transient response arecontrolled through the COMP pin. COMP is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination tocontrol the characteristics of the control system.
The DC gain of the voltage feedback loop is:
V
AGRA×××=
VEACSLOADVDC
Where AG
is the current sense transconductance and
CS
R
is the load resistor value.
LOAD
is the error amplifier voltage gain,
VEA
FB
V
OUT
The system has two poles of importance. One is due to the compensation capacitor (C3) andthe output resistor of error amplifier while theother is dueto the output capacitor and the load resistor. These poles are located at:
G
f××=
1
P
EA
32
π
AC
VEA
In this case, a third pole set by thecompensation capacitor (C6) and thecompensation resistor (R3) is used tocompensate the effectof the ESR zero on theloop gain. This pole is located at:
f
=
3
P
1
π
362
RC
××
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 unity gain) is important.
Lower crossover frequencies result in slowerline and load transient responses,while highercrossover frequencies could cause system instability. A good standard is to set thecrossover frequency to approximately one-tenth of the switching frequency. The switching frequency for the MP1583 is 385KHz, so thedesired crossover frequency is around 38KHz.
Table 3 lists the typical values of compensationcomponents for some standard output voltages with various output capacitors and inductors.The values of the compensation components have been optimized for fast transientresponses and good stability at given conditions.
Where G
=
2
P
is the error amplifier
EA
1
RCf××
22
π
LOAD
transconductance.
The system has one zero of importance, due to the compensation capacitor (C3) and thecompensation resistor (R3). This zero is located at:
f
=
1
Z
1
π
332
RC
××
The system may have another zero of importance, if the output capacitor has a largecapacitance and/or a high ESR value. The zero due to the ESR and capacitance of the outputcapacitor is located at:
To optimize the compensation components for conditions not listed inTable 2, the followingprocedure can be used.
1. Choose the compensation resistor (R3) to set the desiredcrossover frequency. Determine R3by the following equation:
V
f2C2
Where f
3R×
=
is the desired crossover frequency
C
××π
GG
×
CSEA
OUT
C
V
FB
(which typically has a value no higher than38KHz).
2. Choose the compensation capacitor (C3) toachieve the desired phase margin. For applications with typical inductor values, settingthe compensation zero, f
, below one forth of
Z1
the crossover frequency provides sufficient phase margin. Determine C3 by the following equation:
3C
>
4
f3R2
××π
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 atless than half ofthe 385KHz switchingfrequency, or if the following relationship is valid:
PCB Layout Guide
PCB layout is very important to achieve stableoperation. Please follow these guidelines andtake Figure2 and 3 for references.
1) Keep the path of switching current shortand minimize the loop area formed by Inputcap, high-side and low-side MOSFETs.
2) Keep the connection of low-side MOSFETbetween SW pin andinput power ground as short and wide as possible.
3) Ensure all feedback connections are short and direct. Place the feedback resistors and compensation components as close tothe chip as possible.
4) Route SW away from sensitive analog areas such as FB.
5) Connect IN, SW, and especially GND respectively to a large copper area to cool the chip to improve thermal performanceand long-term reliability. For single layer, do not solder exposed pad of the IC
5
FB
R1R2C6
C3
R3
SGND
SGND
C4
R4
8
SS/REFBS
7
EN
6
COMP
1
××π
R2C2
ESR
If this is the case, then add the second compensation capacitor (C6) to set the pole fat the location of the ESR zero. Determine C6 by the equation:
1)Control dimension isin inches. Dimensionin bracket is millimeters.
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 Propertyrights are not infringed upon when integrating MPSproducts into anyapplication. MPS will not assume any legal responsibility for any said applications.