TM
MP2351
1.5A, 23V, 1.4MHz
Step-Down Converter
The Future of Analog IC Technology
TM
DESCRIPTION
The MP2351 is a monolithic step down switch
mode converter with a built in internal power
MOSFET. It achieves 1.5A 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, short circuit frequency foldback
and thermal shutdown. In shutdown mode the
regulator draws 20µA of supply current.
The MP2351 requires a minimum number of
readily available standard external components.
FEATURES
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
1.5A Output Current
0.18Ω Internal Power MOSFET Switch
Stable with Low ESR Output Ceramic
Capacitors
Up to 93% Efficiency
20µA Shutdown Mode
Fixed 1.4MHz Frequency
Thermal Shutdown
Cycle-by-Cycle Over Current Protection
Wide 4.75V to 23V Operating Input Range
Output Adjustable from 1.23V to 16V
Programmable Under Voltage Lockout
Frequency Synchronization Input
Available in QFN (3mm x 3mm) and tiny
10-Pin MSOP Packages
Evaluation Board Available
Distributed Power Systems
Battery Charger
DSL Modems
Pre-Regulator for Linear Regulators
EVALUATION BOARD REFERENCE
Board Number
EV2351DQ-00A
Dimensions
2.3”X x 1.5”Y x 0.5”Z
APPLICATIONS
“MPS” and “The Future of Analog IC Technology” are Trademarks of Monolithic
Power Systems, Inc.
TYPICAL APPLICATION
INPUT
4.75V - 23V
4
2
BS
SW
5
7
C5
10nF
IN
EN
Efficiency vs
Load Current
100
V
OUT
=5V
D1
B220A
EFFICIENCY (%)
OPEN
AUTOMATIC
STARTUP
OPEN
NOT USED
9
10
MP2351
SYNC
GND
6
FB
COMP
8
V
OUT
3.3V/1.5A
90
80
70
60
50
40
0
300 600 900 1200 1500
LOAD CURRENT (mA)
MP2351-EC01
V
OUT
=2.5V
V
OUT
=3.3V
C6
OPEN
C3
1nF
MP2351_TAC_S01
MP2351 Rev. 1.5
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
1
TM
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
PACKAGE REFERENCE
TOP VIEW
NC
BS
NC
IN
SW
1
2
3
4
5
10
9
8
7
6
SYNC
EN
COMP
FB
GND
NC
BS
NC
IN
SW
1
2
3
4
5
TOP VIEW
10
9
8
7
6
SYNC
EN
COMP
FB
GND
EXPOSED PAD
ON BACKSIDE
CONNECT TO
GROUND (PIN 6)
MP2351_PD01-QFN10
MP2351_PD01-MSOP10
Part Number*
MP2351DQ
*
Package
QFN10
(3mm x 3mm)
Temperature
–40°C to +85°C
Part Number*
MP2351DK
*
Package
MSOP10
Temperature
–40°C to +85°C
For Tape & Reel, add suffix –Z (eg. MP2351DQ–Z)
For Lead Free, add suffix –LF (eg. MP2351DQ –LF–Z)
For Tape & Reel, add suffix –Z (eg. MP2351DK–Z)
For Lead Free, add suffix –LF (eg. MP2351DK –LF–Z)
ABSOLUTE MAXIMUM RATINGS
(1)
Supply Voltage (V
IN
)..................................... 25V
Switch Voltage (V
SW
).................................... 26V
Bootstrap Voltage (V
BS
) ....................... V
SW
+ 6V
Feedback Voltage (V
FB
) .................–0.3V to +6V
Enable/UVLO Voltage (V
EN
)...........–0.3V to +6V
Comp Voltage (V
COMP
) ...................–0.3V to +6V
Sync Voltage (V
SYNC
)......................–0.3V to +6V
Junction Temperature .............................+150°C
Lead Temperature ..................................+260°C
Storage Temperature.............. –65°C to +150°C
Recommended Operating Conditions
(2)
Supply Voltage (V
IN
) ...................... 4.75V to 23V
Operating Temperature.................–40°C to +85°C
Thermal Resistance
(3)
QFN10 (3x3)........................... 50 ...... 12...
°C/W
MSOP10 ................................ 150 ..... 65...
°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.
θ
JA
θ
JC
ELECTRICAL CHARACTERISTICS
V
IN
= 12V, T
A
= +25°C, 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
Sync Frequency
MP2351 Rev. 1.5
1/6/2006
Symbol Condition
V
FB
4.75V
≤
V
IN
≤
23V
R
DS(ON)1
R
DS(ON)2
V
EN
= 0V, V
SW
= 0V
G
CS
A
VEA
G
EA
f
S
∆I
C
=
±10
µA
V
FB
= 0V
Sync Drive 0V to 2.7V
Min
1.195
2.4
Typ
1.230
0.18
10
0
2.8
1.95
Max
1.265
10
5.2
Units
V
Ω
Ω
µA
A
A/V
500
1.15
1.6
400
770
1.40
180
1100
1.65
2.1
V/V
µA/V
MHz
KHz
MHz
2
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
TM
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
ELECTRICAL CHARACTERISTICS
(continued)
V
IN
= 12V, T
A
= +25°C, unless otherwise noted.
Parameter
Maximum Duty Cycle
Minimum On Time
EN Shutdown Threshold Voltage
Enable Pull Up Current
EN UVLO Threshold Rising
EN UVLO Threshold Hysteresis
Supply Current (Shutdown)
Supply Current (Quiescent)
Thermal Shutdown
Note:
4) Equivalent output current = 1.5A
≥
50% Duty Cycle
2.0A
≤
50% Duty Cycle
Assumes ripple current = 30% of load current.
Slope compensation changes current limit above 40% duty cycle.
Symbol Condition
D
MAX
V
FB
= 1.0V
T
ON (MIN)
V
FB
= 1.5V
I
CC
> 100µA
V
EN
= 0V
V
EN
Rising
V
EN
≤
0.4V
V
EN
≥
3V, V
FB
= 1.4V
Min
0.7
1.15
2.37
Typ
70
70
1.0
1.50
2.50
210
20
1.1
160
Max
1.3
2.62
36
1.3
Units
%
ns
V
µA
V
mV
µA
mA
°C
PIN FUNCTIONS
Pin #
1
2
Name Description
NC
No Connect.
BS
Bootstrap (C5). This capacitor is needed to drive the power switch’s gate above the supply
voltage. It is connected between 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 MP2351 operates from a +4.75V to +23V 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 run away during a short circuit fault condition the
frequency foldback comparator lowers the oscillator frequency when the FB voltage is below
700mV.
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 V
IN
to GND. For complete low current shutdown it’s the EN
pin voltage needs to be less than 700mV.
SYNC Synchronization Input. This pin is used to synchronize the internal oscillator frequency to an
external source. There is an internal 11kΩ pull down resistor to GND, therefore leave SYNC
unconnected if unused.
3
4
5
6
7
8
9
10
MP2351 Rev. 1.5
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
3
TM
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
OPERATION
The MP2351 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; 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 1.4MHz 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 MP2351 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 1.230V bandgap reference. The polarity is
such that an FB pin voltage lower than 1.230V
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 10Ω 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
SYNC 10
OSCILLATOR
180KHz/
1.4MHz
+
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
--
2.37V/
2.62V
1.8V
+
--
6
GND
+
FREQUENCY
FOLDBACK
COMPARATOR
--
0.7V
7
1.23V
FB
+
ERROR
AMPLIFIER
8
COMP
MP2351_BD01
Figure 1—Functional Block Diagram
MP2351 Rev. 1.5
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
4
TM
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
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:
V
FB
=
V
OUT
R2
R1
+
R2
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
I
LP
=
I
LOAD
+
⎛
V
OUT
V
× ⎜
1
−
OUT
2
×
f
S
×
L
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Where, I
LOAD
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 (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
I
C1
=
I
LOAD
×
V
OUT
⎛
V
OUT
×⎜
1
−
V
IN
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
V
OUT
=
1.23
×
R1
+
R2
R2
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using that value, R1 is determined by:
R1
=
8.18
×
( V
OUT
−
1.23)
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
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
=
⎛
⎞
V
V
OUT
× ⎜
1
−
OUT
⎟
⎜
f
S
×
∆I
L
⎝
V
IN
⎟
⎠
The worst-case condition occurs at V
IN
= 2V
OUT
,
where:
I
C1
=
I
LOAD
2
Where V
IN
is the input voltage, f
S
is the
switching frequency and
∆I
L
is the peak-to-peak
inductor ripple current.
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
MP2351 Rev. 1.5
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
5
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