MP4570
3A, 4.5V-55V Input, Frequency Programmable,
Fully Integrated Synchronous, Step-Down Converter
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP4570 is a frequency programmable stepdown switching converter with integrated internal
high-side and low side power MOSFETs. It can
provide 3A continuous output current with peak
current control for excellent transient response
and efficiency performance.
The wide 4.5V to 55V input voltage range
accommodates a variety of step down
applications, including those applications in
industrial, PoE, automotive and printer with DC
high voltage bus.
The MP4570 uses peak current mode control to
regulate the output voltage. The chip provides
over current protection with valley current
detection which is used to avoid current running
way. Also it has accurate and reliable over
voltage protection, and auto recovery thermal
protection. In addition, the optional external soft
start is available. Enable and power good
indication function can be used to power track
easily. In order to increase the efficiency,
MP4570 will automatically scaling down the
switching frequency when load is light.
Meanwhile, the low side MOSFET will be turned
off to reduce driver loss when zero inductor
current is detected. MP4570 also features as a
fully integrated solution with both high side and
low side power MOSFETs. Synchronous
operation mode with integrated low side
MOSFET is much helpful to reduce the
conduction loss and also beneficial to reduce
external components space and save the cost.
Wide Input Voltage Range: 4.5V to 55V
90mΩ and 70mΩ Internal High and Low
Power MOSFETs
Peak Current Mode Control
Programmable Switching Frequency
Stable Independent on Output Capacitors
Optional External Soft Start
OCP Protection with Valley Current
Detection
Support External SYNC Clock
OVP Protection
Current Limit Decreasing during Output
Short for Better Thermal Performance
Power Good Indication
Thermal Shutdown Protection
Available in a TSSOP-20 EP Package.
APPLICATIONS
PoE Input Non-isolated Buck
Industrial Power Systems
Printers and Scanners
Automotive Power Systems
Distributed Power Systems
All MPS parts are lead-free, halogen free, and adhere to the RoHS
directive. For MPS green status, please visit MPS website under Quality
Assurance.
“MPS” and “The Future of Analog IC Technology” are Registered
Trademarks of Monolithic Power Systems, Inc.
The MP4570 is available in a TSSOP-20 EP with
exposed pad package.
MP4570 Rev. 1.01
1/4/2017
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1
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL APPLICATION
Efficiency vs.
Load Current
100
L1
C1
OFF ON
R5
VDD
R4
C4
VIN
SW
EN
BST
PG MP4570 FB
FREQ
SS
BIAS
VDD
C5
COMP
GND
VOUT=5V
95
C2
C3
R1
VIN=12V
90
85
5V_EX or NC
R3
R2
80
75
70
65
C6
60
VIN=36V
VIN=24V
VIN=48V
VIN=55V
55
50
10
MP4570 Rev. 1.01
1/4/2017
100
1000
10000
LOAD CURRENT (mA)
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2
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
ORDERING INFORMATION
Part Number*
MP4570GF
Package
TSSOP-20 EP
Top Marking
See Below
* For Tape & Reel, add suffix –Z (e.g. MP4570GF–Z);
TOP MARKING
MP4570: product code of MP4570GF;
MPS: MPS prefix:
YY: year code;
WW: week code:
LLLLLLLLL: lot number;
PACKAGE REFERENCE
1
20 AGND
FB
2
19 SS
FREQ
3
18
EN
4
17 BIAS
BST
5
16
VIN
6
15 GND
VIN
7
14 GND
VIN
8
13
GND
NC
9
12
NC
SW
10
11 SW
COMP
PG
VDD
TSSOP-20 EP
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
ABSOLUTE MAXIMUM RATINGS (1)
Thermal Resistance
Supply Voltage VIN ...................................... 60V
VSW ..................................... –0.5V to (VIN+0.5V)
VBS ....................................................... VSW + 6V
All Other Pins ................................. –0.3V to +6V
EN Sink Current ....................................... 150μA
Continuous Power Dissipation
(TA = 25°C) (2)
TSSOP-20 EP .......................................... 2.78W
Junction Temperature ...............................150°C
Lead Temperature ...................................260°C
Storage Temperature .............. –65°C to +150°C
TSSOP-20 EP ......................... 45 ...... 10 ... °C/W
Recommended Operating Conditions
Supply Voltage VIN .......................... 4.5V to 55V
Output Voltage VOUT ........................1V to 0.9·VIN
Operating Junction Temp. (TJ). -40°C to +125°C
MP4570 Rev. 1.01
1/4/2017
(3)
θJA
θJC
Notes:
1) Absolute maximum ratings are rated under room
temperature unless otherwise noted. Exceeding these
ratings may damage the device.
2) The maximum allowable power dissipation is a function of
the maximum junction temperature TJ (MAX), the junctionto-ambient thermal resistance θJA, and the ambient
temperature TA. The maximum allowable continuous
power dissipation at any ambient temperature is
calculated by PD (MAX) = (TJ (MAX)-TA)/θJA. 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) Measured on JESD51-7, 4-layer PCB.
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
ELECTRICAL CHARACTERISTICS
VIN = 48V, VEN = 3.3V, TJ= +25C, unless otherwise noted.
Parameter
Error Amplifier
Symbol Condition
Feedback Voltage
VFB
4.5V ≤ VIN ≤ 55V
FB Current
Error AMP Transconductance
COMP Sink/Source Current
IFB
VFB=1.07V
ICOMP
Min
Typ
Max
Units
0.98
1
1.02
V
430
13
10
630
30
50
730
52
nA
μA/V
μA
Switch Characteristic
Upper Switch On Resistance
RON_HS
90
120
mΩ
Lower Switch On Resistance
RON_LS
70
100
mΩ
Upper Switch Leakage
ILKG_SW
10
300
nA
5.7
7.5
A
450
7
550
12
µA
µA
3.8
V
VEN = 0V, VSW = 0V
Current Limit
Peak Current Limit
ILIMIT
10% Duty Cycle
4.5
IQ
ISHDN
No load, Without Switching
VEN = 0V
VDD Regulator Output Voltage
VDD
BIAS = NC
3.4
3.6
VDD Regulator Output Voltage
VDD
BIAS = External 5V Power
4.6
4.8
Quiescent Supply
Quiescent Supply Current
Shutdown Supply Current
VDD Regulator
V
Threshold Voltage
EN Rising Threshold
VEN_R
1.4
1.6
1.8
V
EN Falling Threshold
VEN_F
1.1
1.3
1.5
V
EN Threshold Hysteresis
VEN_HYS
VIN UVLO Rising Threshold
VIN UVLO Falling Threshold
VIN UVLO Threshold Hysteresis
300
VINUV R
VINUV F
VINUV HYS
mV
3.7
3.3
3.9
3.5
400
4.1
3.7
V
V
mV
2.5
4
5.5
μA
Soft Start
External Soft Start
Charging Current
PWM Comparator
Minimum Off Time (4)
Minimum On Time (4)
MP4570 Rev. 1.01
1/4/2017
Capacitor
ISS
tOFF MIN
tON_MIN
VSS=1V
100
90
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ns
ns
5
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
ELECTRICAL CHARACTERISTICS (continued)
VIN = 48V, VEN = 3.3V, TJ= +25C, unless otherwise noted.
Parameter
Oscillator Frequency
Symbol
Switching Frequency
Condition
Min
Typ
Max
Units
fSW
RFREQ=100k
400
520
640
kHz
VOVP
VFB(OVP)/VFB
108
115
122
%
VOUT Rising, VFB(PG)/VFB
86
90
94
VOUT Falling, VFB(PG)/VFB
81
85
89
OVP Protection
Output OVP Threshold
Power Good
Power Good Threshold
VPG_TH
Power Good Hysteresis
VPG_HYS
Power Good Delay
tPG_DL
5
ΔVFB(PG)/VFB
%
%
VOUT Rising
8
22
37
μs
VOUT Falling
8
21
33
μs
10
100
nA
1000
kHz
Frequency SYNC
SYNC Leakage Current
ILKG_SYNC
SYNC Frequency Range
fSYNC
100
TSD
150
Thermal
Thermal Shutdown (4)
Thermal Shutdown Hysteresis
(4)
TSD_HYS
170
°C
10
°C
Note:
4)
Derived from bench characterization. Not tested in production.
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
PIN FUNCTIONS
Pin #
Name
Description
Compensation Networks Setting. Please connect an external resistor series with a
capacitor between this pin and GND.
Feedback. This is the input to the PWM comparator. Please put an external resistive
divider connected between the output and GND.
Switching frequency setting pin. Connect a resistor from this pin to GND to set the
switching frequency. If external SYNC clock is applied to this pin, the converter will
follow this SYNC clock frequency.
Enable Input. Pulling this pin below the specified threshold shuts the chip down.
There is no internal pull-up or pull-down circuit, so do not float the pin.
Bootstrap. This is the positive power supply for the internal floating high-side
MOSFET driver. Connect a capacitor between this pin and SW pin.
Input Supply. This supplies power to all the internal control circuitry, VDD regulator. A
decoupling capacitor to ground must be placed close to this pin to minimize switching
spikes.
1
COMP
2
FB
3
FREQ
4
EN
5
BST
6,7,8
VIN
9,12
NC
Not connected pin. Please float this pin in the application.
10,11
13,14,
15, Exposed
Pad
SW
Switch Node. This is the output node from the internal high-side MOSFET source.
GND
Power ground pin for internal power MOSFETs.
VDD
Power for internal MOSFET driver and BST charging circuit.
17
BIAS
For better thermal performance, please connect this pin to external 5V source, VDD
and internal circuit will be powered by BIAS. Since there is a diode inside between
the BIAS pin and the internal circuit, please float this pin or connected to GND if not
used.
18
PG
Power good indication. Please connect a resistor to a pull up power source if used.
19
SS
Optional external soft-start time setting. Connect an external capacitor between this
pin and GND to set soft-start time externally. Floating the pin will activate the internal
0.5ms soft-start setting.
20
ANGD
16
MP4570 Rev. 1.01
1/4/2017
Ground for internal logic and signal circuit.
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL CHARACTERISTICS
VIN=48V, unless otherwise noted.
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL CHARACTERISTICS
VIN=48V, unless otherwise noted.
MP4570 Rev. 1.01
1/4/2017
www.MonolithicPower.com
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9
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 48V, VOUT =3.3V, COUT = 2x22µF, L = 10µH, fSW=500kHz, TA = +25C, unless otherwise noted.
Efficiency vs.
Efficiency vs.
Load Regulation
Load Current
Load Current
100
VOUT=3.3V
VOUT=5V
100
VIN=12V
95
95
90
90
85
85
80
75
VIN=36V
70
VIN=24V
75
65
VIN=55V
60
0.02
VIN=48V
0.01
80
70
VIN=48V
65
VIN=24V
0.03
VIN=12V
0.00
VIN=48V
VIN=36V
VIN=55V
60
55
-0.02
55
50
10
100
1000
10000
LOAD CURRENT (mA)
50
10
VIN=12V
-0.01
1000
LOAD CURRENT (mA)
-0.03
1
1.5
2
2.5
LOAD CURRENT(A)
3
Line Regulation
0.10
0.08
IOUT=2A
0.06
IOUT=1A
0.04
0.02
0.00
IOUT=3A
-0.02
-0.04
-0.06
-0.08
-0.10
0
10
20
30
40
50
60
INPUT VOLTAGE (V)
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 48V, VOUT =3.3V, COUT = 2x22µF, L = 10µH, fSW=500kHz, TA = +25C, unless otherwise noted.
MP4570 Rev. 1.01
1/4/2017
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11
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 48V, VOUT =3.3V, COUT = 2x22µF, L = 10µH, fSW=500kHz, TA = +25C, unless otherwise noted.
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
FUNCTIONAL BLOCK DIAGRAM
Figure 1 — Functional Block Diagram
MP4570 Rev. 1.01
1/4/2017
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
OPERATION
The MP4570 is a step-down switching regulator
with integrated high-side high voltage power
MOSFETs. It features as wide input voltage
range, high efficiency, external internal soft-start,
frequency programmable and comprehensive
protection mode, i.e. OVP, OCP, OTP.
PWM Control
The MP4570 uses peak current mode control to
regulate the output voltage.
A PWM cycle is initiated by the internal clock at
the beginning of every cycle. After the high side
MOSFET turns on, the inductor current will rise
linearly to provide the energy to the load. The
high side MOSFET remains on until its current
hits the COMP voltage which is the output of the
internal error amplifier (EA). The output voltage
of error amplifier depends on the difference of
output feedback voltage and the internal high
precision reference and it will decide how much
energy should be transferred to the load. The
higher load current, the higher COMP voltage.
After the high switch is off, the low side switch is
on and the inductor current will flow through the
low side switch. In order to avoid shoo-through
issue, the dead time is inserted to avoid the high
side and low side MOSFETS to turn on at the
same time. For each turn on and off in a
switching cycle, the high side MOSFET will keep
on and off with minimum on and off time limit.
Light Load Operation
In order to get high efficiency, MP4570 has two
features during light load: 1).When the load
current decreases, the inductor current will drop
at same time. The low side MOSFET will turn off
in order to save driver loss when inductor current
drops to zero. 2) When the load decreases, the
switching frequency will be scaled down in order
to reduce switching loss after COMP voltage
drops down lower than certain threshold.
Error Amplifier
The error amplifier compares the FB pin voltage
with the internal reference and outputs a current
proportional to the difference between the two.
This current is used to charge the external
compensation networks to form the COMP
voltage, which is used to control the high side
MOSFET peak current and to regulate the output
voltage.
MP4570 Rev. 1.01
1/4/2017
Oscillator and SYNC Function
The internal oscillator frequency is set by a single
external resistor (RFREQ) connected between
FREQ pin and GND. The frequency setting
resistor should be located close to the device.
The relationship between oscillator frequency
and RFREQ refer to table 1 in APPLICATION
INFORMATION section.
During light load, the switching frequency will be
scalded down according to the COMP voltage.
The switching frequency will start to decrease
when the COMP voltage is lower than around
0.8V. And the switching will be disabled when the
COMP voltage drops lower than around 0.7V.
In order to reduce the switching loss and the
thermal dissipation, the switching frequency will
be decreased according to the FB voltage. When
the FB is lower than 25%xRFE, the switching
frequency starts to decrease from the normal
value, and finally drops to 5% of the normal value
when the FB is zero.
The FREQ pin can be used to synchronize the
internal oscillator rising edge to an external clock
falling edge. Make sure the HIGH amplitude of
SYNC clock is higher than 1.5V and LOW
amplitude is lower than 1V to drive the internal
logic. The recommended external SYNC
frequency is in the range of 100kHz and 1MHz.
There is no pulse width requirement but please
note that there is always parasitic capacitance of
the pad there, so, if the pulse width is too short, a
clear rising and falling edge may not be seen due
to the parasitic capacitance. The pulse longer
than 100ns is recommended in application.
EN Control
EN is a control pin that turns the regulator on and
off: Drive EN higher than 1.6V to turn on the
regulator, drive it lower than 1.3V to turn off.
There is no internal pull-up or pull-down at EN,
so when it is floating, the EN status is uncertain.
The EN pin is clamped internally using a 6.5V
zener diode between EN and GND, as shown in
the functional block diagram. Connecting the EN
pin directly to a voltage source without any pull-
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
up resistor requires limiting voltage amplitude to
≤6V to prevent damage to the zener diode. EN
pin can be connected to higher voltage (e.g. VIN)
through pull-up resistor if the system doesn’t
have another logic signal acting as enable signal.
Just make sure the pull-up resistor is high
enough to make sure the sink current into EN pin
less than 150uA to avoid damaging the zener
diode. For example, when connecting EN to
VIN=12V, RPULL-UP ≥ (12V – 6.5V) ÷ 150µA = 37kΩ.
Soft Start
The soft start is implemented to ensure the
smooth up output voltage during the power on
and off. In addition, the soft start function also
helps to reduce the inrush current value at the
startup.
The soft start function is achieved by ramping SS
up slowly and using SS to override the internal
reference (REF) when SS-900mV is lower than
REF. When SS-900mV is higher than REF, REF
regains the control. The 900mV above is the
offset voltage of SS which means SS is detected
as 0 internally when it is lower than 900mV. To
minimize the delay for SS to reach 0.9V, an
internal pull-up circuit with about 30uA average
current pulls SS up to 600mV first. Then use 4uA
constant current to charge SS until it reaches to
2.5V. When SS is in the range of 0.9V to 1.9V, it
overrides the REF as reference voltage of the
error amplifier. During this period, output voltage
ramps up from 0 to the regulated value following
SS rising. The soft start time (tSS) set by the
external SS capacitor can be calculated by below
formula:
t SS (ms)
C SS (nF) VREF (V)
ISS ( μA)
Where CSS is the external SS capacitor, VREF is
the internal reference voltage (1V), and ISS is the
4 μ A SS charge current.
The delay time for SS reaches to 900mV can be
estimated as below:
t SS _delay (ms)
C SS (nF) 0.6V C SS (nF) 0.3V
30 μA
4 μA
There is also an internal fixed 500us soft start.
The final SS time is determined by the longer
time between 500μs and external SS setting time.
MP4570 Rev. 1.01
1/4/2017
When output voltage is shorted to GND,
feedback voltage is pulled to low then the SS will
be discharged. The part will soft start again when
the short at output is removed.
Internal Regulator and BIAS
There is an internal 2.5V regulator powers all the
internal control circuits. The 2.5V regulator takes
VIN as power supply when BIAS pin is lower than
3.2V while supplies from BIAS when BIAS is
higher than 3.2V.
VDD regulator powers LS driver and also BST
regulator when it is higher than 4.5V. VDD is
powered from VIN when BIAS is floating and
regulated at 3.6V. When BIAS is higher than
4.2V, it starts to power VDD. VDD will increases
as BIAS rising with 600mV voltage drop and
regulated at 4.8V when BIAS is higher than 5.4V.
A 1µF decoupling capacitor is needed at the pin
and make the capacitor as close to the pin as
possible.
Using BIAS to power internal regulator can
improve the efficiency. It is recommended to
connect BIAS to an external power supply that is
in the range of 3.3V to 5.5V. Output voltage is a
good choice for this power supply if it is in above
range. A 0.1µF to 1µF decoupling capacitor at
the pin is recommended.
Over Voltage Protection
The MP4570 monitors the feedback output
voltage to achieve the over voltage protection. If
the FB voltage is higher than the 115%xREF, the
high side and low side MOSFET will be turned off
immediately, the PG signal will be asserted to
inform the fault status. High side and low side
MOSFET will resume switching following close
loop operation after FB voltage drops back to
lower than 103%xREF.
Over Current Protection
The MP4570 has cycle-by-cycle peak current
limit protection and valley current detection
protection. The inductor current is monitored
during the high side MOSFET on state. If the
inductor current exceeds the current limit value
set by COMP voltage, the high side MOSFET
turns off immediately. Then, the low side
MOSFET will be turned on to discharge the
energy and inductor current will decrease. The
high side MOSFET will not be on again unless
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
the inductor valley current is lower than a certain
current threshold which is called the valley
current limit. It is very useful to avoid the inductor
current to run away. Also, both the peak current
limit and the valley current limit value are
dependent on the FB voltage. If the feedback
output voltage is higher than the 50%xREF, the
current limit value is as the normal value. If the
feedback output voltage is lower than 50%xRFF,
the current limit will decrease and drop to the half
normal value when the feedback output voltage
is zero. This feature is very helpful to reduce the
OCP thermal dissipation which may especially
get worse when the output voltage is shorted.
Also, it is very helpful to reduce the high inrush
current during the startup.
UVLO Protection
The MP4570 has input under voltage lockout
protection (UVLO). Assuming the EN is active,
the MP4570 is powered on when the input
voltage is higher than the UVLO rising threshold
and is powered off when input voltage drops
below the UVLO falling threshold.
Thermal Shutdown Protection
The thermal shutdown is employed in MP4570 by
monitoring the IC temperature internally. If the
junction temperature exceeds the threshold
(typically 170oC), the regulator shuts off and it will
turns on again when the temperature drops
below 160oC. There is a ~10oC hysteresis.
threshold is 2.3V with a hysteresis of 300mV.
The driver’s UVLO is soft-start related: When the
bootstrap voltage hits its UVLO threshold, the
soft-start circuit resets. When bootstrap UVLO is
gone, the reset is off and then the soft-start
process resumes.
The dedicated internal bootstrap regulator
regulates and charges the bootstrap capacitor to
4.2V. When the voltage between the BST and
SW nodes is less than its regulation, a PMOS
pass transistor from VIN to BST turns ON. The
charging current path is from VIN, BST and then
to SW.
As long as VIN is sufficiently higher than VSW, the
bootstrap capacitor can be charged. When the
high side MOSFET is ON, VIN≈VSW so the
bootstrap capacitor can’t be charged. When low
side MOSFET is ON, the difference between VIN
and VSW is at its largest, thus making it the best
period to charge. When there is no current in the
inductor, VSW=VOUT so the difference between VIN
and VOUT can charge the bootstrap capacitor.
At higher duty cycles, the time period available
for bootstrap charging is shorter so the bootstrap
capacitor may not be sufficiently charged. If the
internal circuit does not have sufficient voltage
and the bootstrap capacitor is not charged, extra
external circuitry can be used to ensure the
bootstrap voltage is within the normal operational
region.
Power Good
MP4570 has one power good (PG) pin out to
indicate the normal operation after soft start time.
The PG pin is the open drain of an internal
MOSFET. It should be connected to VDD or
external voltage source through a resistor (i.e.
100kohm). After the input voltage is applied, the
MOSFET is turned on and the PG pin is pulled to
GND before SS is ready. After the FB voltage
reaches 90% REF voltage, the MOSFET turns off
and PG pin is pulled to high by external voltage
source. When the FB voltage drops to 85% REF
voltage, the PG voltage is pulled to GND to
indicate a failure output status.
Floating Driver and Bootstrap Charging
An external bootstrap capacitor (0.1µF typically)
between BST pin and SW pin powers the floating
power MOSFET driver. This floating driver has its
own UVLO protection. This UVLO’s rising
MP4570 Rev. 1.01
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16
MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Switching Frequency
The MP4570 has an externally adjustable
frequency. The switching frequency (fSW) can be
set using a resistor at FREQ pin (RFREQ). The
recommended RFREQ values for various fSW see
table1. Refer to fSW vs. RFREQ curve in TYPICAL
CHARACTERISTICS section for more detailed
values.
Table 1 — fSW vs. RFREQ
fSW (kHz)
RFREQ (kΩ)
1000
900
800
700
600
500
400
300
200
100
47.5
56
63.4
73.2
84.5
102
133
178
261
523
Setting the Output Voltage
A resistive voltage divider from the output voltage
to FB pin sets the output voltage. The voltage
divider divides the output voltage down to the
feedback voltage by the ratio:
VFB =VOUT
R2
R1+R2
Thus the output voltage is:
VOUT =VFB
R1+R2
R2
For example, the value for R1 can be 10kΩ. With
this value, R2 is:
R2
10
VOUT 1
lower ripple current that will lower the output
ripple voltage. However, a larger inductor value
will be physically larger, have higher series
resistance, or lower saturation current.
To determine the inductance, allow the inductor’s
peak-to-peak ripple current to approximately
equal 30% of the maximum switch current limit.
Make sure that the peak inductor current is less
than the maximum switch current limit. The
inductance value can be calculated by:
L1
VOUT
fSW ∆IL
V
1 OUT
VIN
Where VOUT is the output voltage, VIN is the input
voltage, fS is the switching frequency, and ∆IL is
the peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
ILP ILOAD
V
VOUT
1 OUT
2 fSW L1
VIN
Where ILOAD is the load current.
Input Capacitor
The input current to the step-down converter is
discontinuous and requires a capacitor to supply
the AC current to the step-down converter while
maintaining the DC input voltage. Use capacitors
with low equivalent series resistances (ESR) for
the best performance. Ceramic capacitors are
best, but tantalum or low-ESR electrolytic
capacitors may also suffice.
For simplification, choose the input capacitor with
an RMS current rating greater than half of the
maximum load current. The input capacitor (C1)
can be electrolytic, tantalum, or ceramic.
kΩ
So for a 3.3V output voltage, R1 is 10kΩ, and R1
is 4.32kΩ.
Inductor
The inductor provides constant current to the
output load while being driven by the switched
input voltage. A larger-value inductor will result in
MP4570 Rev. 1.01
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
When using electrolytic or tantalum capacitors,
place a small, high-quality, ceramic capacitor
(0.1μF) 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 the
input. The input voltage ripple caused by
capacitance is approximately:
∆VIN
V
ILOAD
V
OUT 1 OUT
fSW C1 VIN
VIN
Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or lowESR electrolytic capacitors. Low-ESR capacitors
are preferred to keep the output voltage ripple
low. The output voltage ripple can be estimated
as:
∆VOUT
VOUT VOUT
1
fSW L
VIN
1
R ESR
8
f
C2
SW
Where L is the inductor value and RESR is the
ESR value of the output capacitor.
For ceramic capacitors, the capacitance
dominates the impedance at the switching
frequency and contributes the most to the output
voltage ripple. For simplification, the output
voltage ripple can be estimated by:
∆VOUT
V
VOUT
1 OUT
VIN
8 f L C2
2
SW
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. For simplification, the output ripple is
approximately:
∆VOUT
VOUT VOUT
1
fSW L
VIN
R ESR
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP4570 can be optimized for a wide range of
capacitances and ESR values.
Compensation Components
MP4570 employs current-mode control for easy
compensation and fast transient response. The
COMP pin controls the system stability and
MP4570 Rev. 1.01
1/4/2017
transient response. The COMP pin is the output
of the internal error amplifier. A series capacitorresistor
combination
sets
a
pole-zero
combination to control the control system’s
characteristics. The DC gain of the voltage
feedback loop is:
A VDC R LOAD G CS A VEA
VFB
VOUT
Where AVEA is the error-amplifier voltage gain,
is
the
current-sense
1000V/V;
GCS
transconductance, 12A/V; RLOAD is the load
resistor value.
The system has two important poles: One from
the compensation capacitor (C3) and the output
resistor of error amplifier, and the other due to
the output capacitor and the load resistor. These
poles are located at:
fP1
GEA
2π C3 A VEA
fP2
1
2π C2 R LOAD
Where,
GEA
is
the
transconductance, 630μA/V.
error-amplifier
The system has one important zero due to the
compensation capacitor and the compensation
resistor (R3). This zero is located at:
fZ1
1
2π C3 R3
The system may have another significant zero if
the output capacitor has a large capacitance or a
high ESR value. This zero is located at:
fESR
1
2π C2 RESR
In this case, a third pole set by the compensation
capacitor (C4) and the compensation resistor can
compensate for the effect of the ESR zero. This
pole is located at:
fP3
1
2π C4 R3
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MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER
The goal of compensation design is to shape the
converter transfer function for a desired loop gain.
The system crossover frequency where the
feedback loop has unity gain is important: Lower
crossover frequencies result in slower line and
load transient responses, while higher crossover
frequencies lead to system instability. Generally,
set the crossover frequency to ~0.1×fSW.
Follow these steps to design the compensation:
1. Choose R3 to set the desired crossover
frequency:
R3
2π C2 fC VOUT
GEA GCS
VFB
Where fC is the desired crossover frequency.
2. Choose C3 to achieve the desired phase
margin. For applications with typical inductor
values, set the compensation zero (fZ1)
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