MPS MP2905 3v-28v input, hysteretic synchronous step-down controller Datasheet

MP2905
3V-28V Input, Hysteretic
Synchronous Step-Down Controller
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP2905 is a hysteretic voltage-mode control,
synchronous PWM buck controller. The output
voltage can be regulated from 0.6V to 0.9*VIN. It
achieves high output current over a wide input
supply range from 3V-28V.
MP2905 integrates an internal LDO regulator that
makes the controller function at a wide input
supply.
Hysteretic voltage-mode control provides fast
transient response without additional loop
compensation.
An adjustable soft-start prevents inrush current at
turn-on. The device senses high-side switch
voltage drop for hiccup current limit and short
current protection. The frequency is adjustable
from 200kHz to 500kHz.
The MP2905 is available in 10-pin MSOP
package, provides a very compact system
solution with minimal reliance on external
components.
•
•
•
•
•
•
•
Wide 3V to 28V Operating Input Range
Output Adjustable from 0.6V to 0.9*Vin
Switching frequency from 200kHz to 500kHz
Programmable Soft-Start
Hiccup current limit
Lossless peak current sensing
MSOP-10 package
APPLICATIONS
•
•
•
•
•
Motherboard Power Supplies
AGP and PCI-Express Power Supplies
Graphic-Card Power Supplies
Set-Top Boxes
Point-of-Load Power Supplies
TYPICAL APPLICATION
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
ORDERING INFORMATION
Part Number*
Package
Top Marking
Temperature
MP2905EK
MSOP10
2905E
–20°C to +85°C
*For Tape & Reel, add suffix –Z (eg. MP2905EK–Z).
For RoHS compliant packaging, add suffix –LF (eg. MP2905EK–LF–Z)
PACKAGE REFERENCE
TOP VIEW
FB
1
10
ILIM
SS
2
9
IN
REF
3
8
HG
GND
4
7
SW
LG
5
6
BST
ABSOLUTE MAXIMUM RATINGS
(1)
IN to GND ……... ….. …............-0.3V to +30V
REF to GND …………................-0.3V to +6.5V
IN to REF ……........ …................-0.3V to +25V
SS to GND …………..….. -0.3V to (REF + 0.3V)
LG to GND …………….. -0.3V to (REF + 0.3V)
BST to GND…………………….. -0.3V to 36.5V
BST to SW……………..…….….-0.3V to + 6.5V
SW to GND………..………….….-0.3V to +30V
HG to SW …………..….-0.3V to (BST + 0.3) V
FB to GND……………………..…-0.3V to +6.5V
ILIM to GND ......... …..........-0.3V to (IN + 0.3V)
ILIM to SW ………………….-0.6V to (IN + 0.3V)
HG and LG continuous current...±250mA RMS
(2)
Continuous Power Dissipation (TA = +25°C)
……………………………………………….0.77W
Junction Temperature.............................+140°C
Storage Temperature Range
. ……….....................................-65°C to +150°C
Lead Temperature (soldering, 10s)
……........................................................+300°C
MP2905 Rev. 0.91
4/18/2011
Recommended Operating Conditions
(3)
Supply Voltage VIN .............................. 3V to 28V
Operating Temperature ............. –20°C to +85°C
Thermal Resistance
(4)
θJA
θJC
MSOP10-EP ............................. 150 65 °C/W
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. θJA and the ambient temperature,
TA the maximum allowable power dissipation at any ambient
temperature is calculated using: 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. Thermal
o
shutdown engages at TJ=140 C(TYP) and disengages at
o
TJ=120 C(TYP)
3) The device is not guaranteed to function outside of its
operating conditions.
4) Measured on JESD51-7 4-layer board.
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
ELECTRICAL CHARACTERISTICS
VIN=12V, 4.7uF capacitor from REF, 0.01uf capacitor from SS to GND; VFB=0.65V; VSW=VGND=0V;
VILIM=11.5V; HG=unconnected; LG=unconnected; TA=25oC, unless otherwise noted.
Parameter
IN Supply Voltage
Condition
Min
3 (5)
Typ
Max
28
Unit
V
REF output Voltage
IREF=10mA
4.75
5.0
5.25
V
REF maximum output
current
BST output voltage
BST maximum output
current
IBST=10mA
REF
Undervoltage
lockout (UVLO)
Rising
Falling
Hysteresis
Supply Current
No
switching,
VFB=0.65V
Output
accuracy
Output
hysteresis
regulation
20
5.0
2.6
2.25
VIN=12V
VIN=VREF=5V
VIN=VREF=3.3V
VFB peak
0.593
FB falling to LG falling
FB rising to HG falling
OvervoltageProtection(OVP)
Threshold
High-Side Current –
Sense Program Current
Soft-Start
internal
Resistance
Fault Hiccup Internal SS
Pulldown Current
HG Driver Resistance
LG Driver Resistance
Dead time
TA=85oC
TA=25oC
Thermal Shutdown
mA
2.8
2.45
350
0.6
0.7
0.6
3
2.65
V
V
mV
2
2
2
mA
0.6
0.607
V
22
mV
50
70
ns
ns
0.7
0.75
0.8
V
42.5
60
50
57.5
uA
uA
60
80
100
K
VSW<VILIM and VFB<VSS
230
Sourcing Resistance
2.1
Sinking Resistance
1.6
Sourcing Resistance
Sinking Resistance
HG low to LG high and LG low to HG
high(adaptive)
2.2
1.1
Ω
40
ns
HG Minimum On-Time
LG Minimum On-Time
V
20
regulation
FB Propagation Delay
mA
Normal operation
Current fault
Rising temperature, hysteresis=20(typ)
100
100
475
140(6)
nA
Ω
200
200
ns
ns
ns
o
C
Notes:
5) If IN Supply Voltage is lower than 5V, circuit can keep work but the efficiency will be lower.
6) Guaranteed by design
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
PIN FUNCTIONS
Pin#
Name
1
FB
2
SS
3
4
REF
GND
5
LG
6
BST
7
SW
8
HG
9
IN
10
ILIM
MP2905 Rev. 0.91
4/18/2011
Description
Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB with a
resistive voltage divider from the output voltage. The feedback reference voltage is
0.59V. See Setting the Output Voltage.
Soft-start Control Input. SS controls the soft-start period. Connect a capacitor from SS to
GND to set the soft-start period.
A 0.01uF external capacitor sets the soft-start period to 4ms with an internal 84Kohm
resistor.
An internal 250nA current sink in hiccup mode gives approximately 10% duty cycle
during fault conditions.
Internal 5V LDO output. Bypass REF to GND with a 4.7uF or greater capacitor.
Ground. Connect the exposed pad to pin 4
Low-side Gate-Drive output. Drive the synchronous-rectifier MOSFET.
Connect this pin to the gate of the synchronous MOSFET.
High-Side Gate Drive Boost Input. BST supplies the drive for the high-side N-channel
MOSFET switching. Connect a 0.1uF or greater capacitor from SW to BST to power the
high side switch.
MP2905 contains an internal BST regulator, so external schottky from REF to BST is not
necessary.
Switch 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
BST to power the high-side switch.
High-side gate-drive output. Drive the high-side MOSFET. Connect this pin to the gate of
the high-side MOSFET
Power Input. IN supplies the power to the IC, as well as the step-down converter
switches. Drive IN with a 3V to 28V power source. Bypass IN to GND with a suitably
large capacitor to eliminate noise on the input to the IC. See Setting the Input Capacitor.
Current-limit threshold Set pin. A appropriate resistor should be connected between this
pin and the drain of high-side MOSFET (IN). An internal 50uA current sink sets a voltage
drop in the resistor. The voltage drop compares to high-side MOSFET voltage drop (Vds)
to set the peak current-limit threshold.
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN=12V, VOUT =1.8V, IOUT=12A, L1=1.5µH(DCR=3.41mΩ), COUT=100µF *2+330µF, TA=+25°C, High
Side MOS:SI7112DN-T1-E3, Low Side MOS:SI7336ADP-T1-E3 unless otherwise noted.
Efficiency
Supply Current (no switching)
vs. Input Voltage
VFB=0.65V
100
90
0.7
80
0.6
70
0.5
0.4
0.3
0.2
2.00
LOAD REGULATION(%)
0.8
EFFICIENCY(%)
IIN(mA)
0.9
Load Regulation
VIN=5.5V
VIN=12V
60
50
VIN=28V
40
30
20
0.1
10
0
0
0
5
10
15
20
25
30
0
2
VIN (V)
4
6
8
10
12
1.50
VIN=28V
VIN=12V
1.00
0.50
0.00
VIN=5.5V
-0.50
-1.00
-1.50
-2.00
0
2
IOUT (A)
4
6
8
10
12
IOUT (A)
Frequency vs. Input Voltage
Line Regulation
no load, Feedforward Cap=22nF
2.00
300
250
1.00
IOUT=0A
FREQUENCY(kHz)
LINE REGULATION(%)
1.50
IOUT=6A
0.50
0.00
-0.50
-1.00
IOUT=12A
150
100
50
-1.50
-2.00
200
5
10
15
20
VIN (V)
MP2905 Rev. 0.91
4/18/2011
25
30
0
5
10
15
20
25
30
VIN (V)
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN=12V, VOUT =1.8V, IOUT=12A, L1=1.5µH, COUT=100µF *2+330µF, TA=+25°C, unless otherwise
noted.
VIN Power up
VIN Power off
VIN Power up
IOUT=0A
IOUT=0A
IOUT=12A
VOUT
1V/div
VIN
5V/div
VIN
5V/div
VIN
5V/div
VOUT
1V/div
VOUT
1V/div
IINDUCTOR
5A/div
IINDUCTOR
5A/div
SW
10V/div
IINDUCTOR
10A/div
SW
20V/div
1ms/div
SW
10V/div
1s/div
1ms/div
Start up and Shut down by
Pull down and Release SS
VIN Power off
IOUT=12A
VOUT
1V/div
VOUT
10V/div
Short Entry
VOUT
1V/div
VIN
5V/div
SW
10V/div
External Signal
1V/div
IINDUCTOR
10A/div
SS
500mV/div
SW
10V/div
IINDUCTOR
10A/div
IINDUCTOR
10A/div
1ms/div
20ms/div
4ms/div
Short Recovery
FB Peak and Hysteresis
FB
100mV/div
VOUT
1V/div
FB OVP
FB
100mV/div
VOUT
1V/div
SW
10V/div
HG
10V/div
SW
5V/div
IINDUCTOR
10A/div
LG
5V/div
20ms/div
MP2905 Rev. 0.91
4/18/2011
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40ms/div
6
MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN=12V, VOUT =1.8V, IOUT=12A, L1=1.5µH, COUT=100µF *2+330µF, TA=+25°C, unless otherwise
noted.
Pre-bias Test
Output Ripple Voltage
Output Ripple Voltage
Pre-bias Output Voltage=1.7V
IOUT=0A, VOUT_RIPPLE=18mV
IOUT=12A, VOUT_RIPPLE=17.6mV
VOUT/AC
10mV/div
VOUT
500mV/div
VIN
5V/div
VOUT/AC
20mV/div
IINDUCTOR
5A/div
SW
5V/div
HG
10V/div
IINDUCTOR
2A/div
LG
5V/div
2ms/div
SW
5V/div
2us/div
Load Transient Response
Load Transient Response
IOUT=0A~12A@1A/us
IOUT=6A~12A@1A/us
VOUT/AC
100mV/div
VOUT/AC
50mV/div
IOUT
5A/div
IOUT
5A/div
200us/div
MP2905 Rev. 0.91
4/18/2011
2us/div
200us/div
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
FUNCTION DIAGRAM
IN
ILIM
+
-REF
REF REG
R1
R2
BST
BST REG
BG
1.23V
OC COM
+
level shift down
--
R3=84K
SW
SS
SW
R4=80K
GND
0.6V
NOR
--
NAND
BUF
SW
LOOP COMP
FB
HG
level shift up
+
level shift down
+
SW
REF
--
0.75
OVP COM
0.05
OR
LG
+
SS
BUF
--
SHUT DOWN COM
Figure 1—Functional Block Diagram
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
OPERATION
MP2905 uses a hysteretic control loop to
regulate output voltage. It senses the voltage at
FB pin, which is compared with SS voltage with
20mV hysteresis. When FB is lower than SS
20mV, high side switch turns on and FB voltage
rises up. After FB voltage reaches SS voltage,
high side switch turns off and low side switch
turns on, which cause FB voltage drop until FB is
lower than SS 20mV. So Vout is regulated at a
stable voltage because FB voltage is regulated at
the voltage (SS-10mV). See the setting output
voltage for detailed information. Hysteretic
voltage-mode control provides fast transient
response without additional loop compensation.
MOSFET turns off, SS cap will be discharged by
250nA current. After SS voltage is lower than
50mV, SS cap is stopped discharging and high
side switch tries to turn on again.
Soft-start
MP2905 doesn’t need Schottky, but still needs
0.1uF BST cap between BST pin and SW pin.
An external cap is connected at SS pin to realize
soft-start function. When SS pin pull down
transistor turns off, internal reference begins to
charge SS external cap through a resistor-divider.
So FB rises slowly following SS voltage and
inrush current is avoided. Soft-start time is
determined by external cap and internal resistordivider.
If SS has pre-bias voltage at startup, both HG
and LG keep low, SS cap begins to discharge
until lower than 50mV. Then SS will rise slowly
and FB tracks SS.
Startup Sequence
In MP2905, at startup, if FB>SS, which means
output has pre-bias voltage, HG and LG don’t
toggle until SS greater than FB.
Current Limit Function
A resistor is connected from the Drain of the high
side MOSFET to ILIM pin to set current limit
value. Internal 50uA current sink from ILIM to
GND limits the maximum VDS cross high side
switch drain and source. When VFB<300mV, if
high side switch current hits the current limit, high
side switch turns off immediately. If VFB>300mV,
over current event occurs in four sequential
cycles, high side switch turns off. Once high side
MP2905 Rev. 0.91
4/18/2011
Output Over-voltage protection
Output over-voltage is monitored by FB voltage.
If FB voltage higher than 750mV, HG is set low
and LG is set high. This status will be latched
until restart EN or VIN.
Enable
MP2905 turns off by pulling down SS pin to lower
than 50mV. Releasing SS will start the run cycle.
High Side Gate Driver
Internal Regulator
Most of the internal circuitries are powered from
the 5V internal regulator (REF). This regulator
takes VIN input and operates in the full input
range. When VIN is greater than 5.0V, the output
of the regulator is full regulation. When VIN is
lower than 5.0V, the output decreases. Bypass
REF pin to GND with a 4.7uF or greater capacitor.
Under Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented to
protect the chip from operating at insufficient
supply voltage.
The MP2905 UVLO comparator monitors the
output voltage of the internal regulator (REF).
Thermal protection
The purpose of thermal protection is to prevent
damage in the IC by allowing exceptive current to
flow and heating the junction. The die
temperature is internally monitored until the
thermal limit is reached. Once this temperature is
reached, the part will be shut down and allow the
chip to cool. When the IC is cool enough, the part
will be turned on again. There is a built-in
hysteresis.
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see the schematic on front
page).
R1 is for approximately 50µA to 150µA bias
current in the resistor-divider. A wide range of
resistor R1 value is acceptable, choosing a
typical value 6.04k, R3 is determined by:
R3 = R1× (
Setting Current Limit
MP2905 current limit can be set by an external
resistor (R2) which is connected between ILIM
pin and the drain of the high side MOSFET. An
internal 50uA sink current sets a voltage drop
on the resistor. The voltage drop compares to
high-side MOSFET voltage drop (Vds) to set
the peak current limit threshold. Below is the
diagram of current limit function:
Vout + 0.01V + (RDC × 0.5 × ILOAD )
− 1)
VFB
where VFB = 0.590V, RDC is the DC resistance
of the output inductor, ILOAD is the full load
current, 0.5*ILOAD is half load condition, it’s for
Load regulation standard. The term 0.01V is to
reflect 1/2 of the feedback threshold hysteresis.
But R3 value also can’t be too large, or circuit
may work abnormally.
ILIM
RILIM
--
VIN
+
+
Rdson_max
--
OC COM
+
High Side
V-
--
Ids
Switch
V+
SW
SW
Selecting the 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 is favorable to less ripple current
and the lower output ripple voltage. However,
the larger value inductor will have a larger
physical size, higher series DC 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:
L1 =
VOUT
V
× (1 − OUT )
fS × ∆IL
VIN
where VIN is the input voltage, VOUT is the output
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:
VOUT
V
ILP = ILOAD +
× (1 − OUT )
2 × fS × L1
VIN
where ILOAD is the full load current.
MP2905 Rev. 0.91
4/18/2011
Figure 2—Current Limit Functional Diagram
The voltage drop on the high side MOSFET is:
VDS _ ON (max) = IDS (max) × RDS _ ON (max)
Where IDS(max) equals the max peak inductor
current ILP (max).
Then, RILIM can be calculated using the
VDS_ON(MAX) with the following formula:
RILIM =
VDS _ ON (max)
50uA
(Ω)
And RILIM should be kept in the range of 1kΩ~
8kΩ.
Selecting Power MOSFETs
The MP2905 connects two external, logic-level,
n-channel MOSFETs as the circuit switching
elements. The MOSFETs are the key points for
circuit efficiency. The major parameters we
should concern are:
1) On-resistance, RDS_ON: the lower, the better it
will be.
2) Continuous Drain Current (@10sec), ID: it
should be higher than the peak current @ full
load condition. And pay attention to ID’s
variation with different temperature.
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
3) Maximum drain-to-source voltage, VDS(MAX):
it should be at least 20% higher than the input
supply rail at the high-side MOSFET’s drain.
Except the losses above, there still is output
cap loss in both high side MOSFET and low
side MOSFET. Output cap loss is defined by:
4) Total gate charge Qg: the lower, the better it
will be. For high-side MOSFET, the main power
loss consists of conduction loss, switching loss,
and drive loss. The high-side MOSFET
conduction loss can be calculated by:
1
2
× CDS × VDS
× fS
2
where CDS is the output cap of MOSFET.
For less switching noise, add drive resistors in
series with the gate of MOSFET to slow down
the transition between the high-side MOSFET
and low-side MOSFET switching.
2
Phigh−side _ conduction = ILOAD
× Rhighside _ DSON × D
Where D is the duct cycle, it’s defined by:
V
D = OUT
VIN
High-side MOSFET switching loss is calculated
by:
1
Phigh−side _ switching = VIN × ILOAD × (t ON + t OFF ) × fS
2
Where tON is high-side MOSFET turn on time,
tOFF is high-side MOSFET turn off time, fS is the
switching frequency.
High-side MOSFET drive loss is calculated by:
Phigh−side _ drive = Qg _ high−side × fS × Vdrive
Where Vdrive is the high-side MOSFET driving
voltage, typical value is 5V.
For low-side MOSFET, there isn’t switching loss,
conduction loss is the main loss, so we’d better
choice a MOSFET with lower Rds-on than high
side MOSFET. The recommended Rds-on of
low side MOSFET is one-third of high-side
MOSFET. The low-side MOSFET loss consists
of conduction loss, drive loss and body diode
conduction loss. The Low-side MOSFET
conduction loss is calculated by:
2
Plow −side _ conduction = ILOAD
× Rlow −side _ DSON × (1 − D)
Low-side MOS drive loss is calculated by:
Plow −side _ drive = Qg _ low −side × fS × Vdrive
Body diode conduction loss is calculated by:
Pbodydiode = 2 × VF × ILOAD × t deadtime × fS
Where VF is body diode forward voltage drop,
tdeadtime is high-side MOSFET and low-side
MOFETS transition time.
MP2905 Rev. 0.91
4/18/2011
PCds =
Selecting the Feed Forward Capacitor
The feed forward capacitor (C8 in front page
typical application circuit) is a key factor to
affect the frequency. It can be calculated by:
fS ==
1
×
RFB × C8
V
VFB
× (1 − O )
VIN
VFB
1
VIN
VH +
(90ns ×
− 20ns ×
)
C8
R3
RFB
Where fS is desired the frequency, VFB is
feedback reference voltage, typical is 590mV,
VH is output regulation hysteresis, typical value
is 22mv, RFB is the equivalent value of two
voltage-divided resistors. For example, in 2905
typical application:
R1 + R3
R1× R3
Select an X7R ceramic capacitor with the
closest Capacitance to the value calculated as
possible. Increase the Capacitance, the
switching frequency decrease, and vice versa,
decrease the Capacitance, the frequency
increase.
RFB =
And output capacitor, inductor and inductor
DCR will affect the frequency, too, but those are
limited.
The frequency calculated by the formula has a
deviation within 30%.
Setting the 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 with X5R or
X7R dielectrics are highly recommended
because of their low ESR and small
temperature coefficients.
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
Since the input capacitor (Cin) 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 ×
VOUT
V
× (1 − OUT )
VIN
VIN
The worse case condition occurs at VIN = 2VOUT,
where
I
ICin = LOAD
2
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. 0.1µF, 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
terminal. The input voltage ripple can be
estimated by capacitance:
I
V
V
D VIN = LOAD ´ OUT ´ (1- OUT )
fS ´ Cin
VIN
VIN
D VOUT =
VOUT
V
´ (1- OUT )
VIN
8 ´ fS ´ L1´ COUT
2
In the case of tantalum or electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency. For simplification, the
output ripple is:
D VOUT =
VOUT
V
´ (1- OUT ) ´ RESR
fS ´ L1
VIN
The characteristics of the output capacitor also
affect the transient response and the stability of
the regulation system. Higher voltage ripple will
influence the voltage of the feed forward
capacitor to make the system be unstable. So,
at the condition which the tantalum or
electrolytic capacitors with higher ESR is used
or output current is higher, a RC filter is
necessary from Vout to GND. Connect the
resistor of filter between the Vout and feed
forward capacitor, and connect the capacitor of
filter from feed forward capacitor to GND.
Follow the R7 and C12 connection in MP2905
typical application. 10Ω/1uF or 2Ω/4.4uF is
recommended for good stability and better
transient response.
Setting the Output capacitor
The output capacitor (Cout) 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:
D VOUT =
VOUT
V
1
´ (1- OUT ) ´ (RESR +
)
fS ´ L1
VIN
8 ´ fS ´ COUT
Where L is the inductor value and RESR is the
equivalent series resistance (ESR) value of the
output capacitor.
In the case of ceramic capacitors, the
impedance at the switching frequency is
dominated by the capacitance. The output
voltage ripple is mainly determined by the
capacitance. For simplification, the output
voltage ripple is:
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
TYPICAL APPLICATION CIRCUITS
Figure 3—Typical Application Circuits for 12A Output
Figure 4—Typical Application Circuits for 25A Output without Droop(7)
Note:
7) For 25A application design, please refer to MPS special application note for 25A application.
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
PCB Layout Guide
PCB layout is very important to achieve stable
operation. It is highly recommended to duplicate
EVB layout for optimum performance.
If change is necessary, please follow these
guidelines:
1) Keep the path of switching current short and
minimize the loop area formed by Input cap,
high-side MOSFET and low-side MOSFET.
2) IC bypass ceramic capacitors are
suggested to be put close to the IN Pin.
3) Ensure all feedback connections are short
and direct. Place the feedback resistors as
close to the 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
improve chip thermal performance and long
term reliability.
6) It is suggested to add the snubber circuit
across the high side MOSFET (IN pin and
SW pin) so as to reduce the SW spike.
7) If output current is higher than 10A,
recommend to use a four layers PCB, and
pour ground in mid layer.
Figure 5—MP2905 Application Circuit
MP2905 Rev. 0.91
4/18/2011
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MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER
Top Layer
IN1
IN2
Bottom Layer
Figure 6—MP2905 Application Circuit and PCB Layout Guide
MP2905 Rev. 0.91
4/18/2011
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MP2905 – PRODUCT DESCRIPTION IN A #-PIN PACKAGE TYPE
PACKAGE INFORMATION
MSOP10
0.114(2.90)
0.122(3.10)
6
10
0.114(2.90)
0.122(3.10)
PIN 1 ID
(NOTE 5)
0.007(0.18)
0.011(0.28)
0.187(4.75)
0.199(5.05)
5
1
0.0197(0.50)BSC
BOTTOM VIEW
TOP VIEW
GAUGE PLANE
0.010(0.25)
0.030(0.75)
0.037(0.95)
0.043(1.10)MAX
SEATING PLANE
0.002(0.05)
0.006(0.15)
FRONT VIEW
0o-6o
0.016(0.40)
0.026(0.65)
0.004(0.10)
0.008(0.20)
SIDE VIEW
NOTE:
0.181(4.60)
0.040(1.00)
0.012(0.30)
1) CONTROL DIMENSION IS IN INCHES. DIMENSION IN BRACKET IS
IN MILLIMETERS.
2) PACKAGE LENGTH DOES NOT INCLUDE MOLD FLASH,
PROTRUSION OR GATE BURR.
3) PACKAGE WIDTH DOES NOT INCLUDE INTERLEAD FLASH OR
PROTRUSION.
4) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING)
SHALL BE 0.004" INCHES MAX.
5) PIN 1 IDENTIFICATION HAS THE HALF OR FULL CIRCLE OPTION.
6) DRAWING MEETS JEDEC MO-817, VARIATION BA.
7) DRAWING IS NOT TO SCALE.
0.0197(0.50)BSC
RECOMMENDED LAND PATTERN
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.
MP2905 Rev. 0.91
4/18/2011
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