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MP2451DT-LF-Z

P1-P3P4-P6P7-P9P10-P12P13-P15
MP245136V, 2MHz, 0.6A, STEP-DOWN CONVERTER
MP2451 Rev. 1.33 www.MonolithicPower.com
4/29/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
7
When COMP voltage is higher than the sleep
threshold, the PAUSE signal is reset so the chip
is back into normal PWM operation. Every time
when the PAUSE changes states from low to
high, a turn-on signal is generated right away,
turning on the power MOSFET.
Error Amplifier
The Error amplifier is composed of an internal
OP-AMP with an R-C feedback network
connected between its output node (internal
COMP node) and its negative input node (FB).
When FB is lower than its internal reference
voltage (REF), the COMP output is then driven
higher by the OP-AMP, causing higher switch
peak current output hence more energy delivered
to the output. Vise versus.
When connecting to the FB pin, normally there is
a voltage divider composed of R
UP
and R
DN
where R
DN
is between FB and GND while R
UP
is
between the voltage output node and FB. R
UP
serves also to control the gain of the error
amplifier along with the internal compensation R-
C network.
Internal Regulator
Most of the internal circuitry is powered on by the
2.6V internal regulator. This regulator takes V
IN
input and operates in the full V
IN
range. When
VIN is greater than 3.0V, the output of the
regulator is in full regulation. When V
IN
is lower,
the output degrades.
Enable Control
The MP2451 has a dedicated enable control pin
EN. With high enough V
IN
, the chip can be
enabled and disabled by EN pin. This is a HIGH
effective logic. Its rising threshold is 1.55V
typically and its trailing threshold is about 300mV
lower. When floating, EN pin is internally pulled
down to GND so the chip is disabled.
When EN is pulled down to 0V, the chip is put
into the lowest shutdown current mode. When
EN is higher than zero but lower than its rising
threshold, the chip is still in shutdown mode but
the shutdown current increases slightly.
Internally a zener diode is connected from EN pin
to GND pin. The typical clamping voltage of the
zener diode is 7.5V. So V
IN
can be connected to
EN through a high ohm resistor if the system
doesn't have another logic input acting as enable
signal. The resistor needs to be designed to limit
the EN pin sink current less than 100μA.
Under Voltage Lockout (UVLO)
V
IN
Under voltage lockout (UVLO) is implemented
to protect the chip from operating at insufficient
supply voltage. The UVLO rising threshold is
about 2.9V while its trailing threshold is about
400mV lower.
Internal Soft-start
Reference type soft-start is implemented to
prevent the converter output voltage from
overshooting during startup. When the chip starts,
the internal circuitry generates a soft-start voltage
(SS) ramping up from 0V at a slow pace set by
the soft-start time. When it is lower than the
internal reference REF, SS overrides the REF so
the error amplifier uses SS instead of REF as the
reference. When SS is higher than REF, REF
gains the control back.
SS is also associated with FB. Though SS can
be much lower than FB, it can only be slightly
higher than FB. If somehow FB is brought down,
SS follows to track FB. This function is designed
to accommodate the short-circuit recovery
situation. When a short-circuit is removed, the SS
ramps up as if it is a fresh soft-start process. This
prevents output voltage overshoot.
Thermal Shutdown
Thermal shutdown is implemented to prevent the
chip from thermally running away. When the
silicon die temperature is higher than its upper
threshold, it shuts down the whole chip. When
the temperature is lower than its lower threshold,
thermal shutdown is gone so the chip is enabled
again.
Floating Driver and Bootstrap Charging
The floating power MOSFET driver is powered by
an external bootstrap capacitor. This floating
driver has its own UVLO protection. This UVLO’s
rising threshold is about 2.4V with a threshold of
about 300mV. During this UVLO, the SS voltage
of the controller is reset to zero. When the UVLO
is removed, the controller follows soft-start
process.
MP245136V, 2MHz, 0.6A, STEP-DOWN CONVERTER
MP2451 Rev. 1.33 www.MonolithicPower.com
4/29/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
8
The bootstrap capacitor is charged and regulated
to about 5V by the dedicated internal bootstrap
regulator. When the voltage between BST and
SW nodes is lower than its regulation, a PMOS
pass transistor connected from V
IN
to BST is
turned on. The charging current path is from V
IN
,
BST and then to SW. External circuit should
provide enough voltage headroom to facilitate the
charging.
As long as V
IN
is sufficiently higher than SW, the
bootstrap capacitor can be charged. When the
power MOSFET is ON, V
IN
is about equal to SW
so the bootstrap capacitor cannot be charged.
When the external free wheeling diode is on, V
IN
to SW difference is the largest so it is the best
period to charge. When there is no current in the
inductor, SW equals to the output voltage V
OUT
so
the difference between V
IN
and V
OUT
can be used
to charge the bootstrap capacitor.
At higher duty cycle operation condition, the time
period available to the bootstrap charging is less
so the bootstrap capacitor may not be charged
sufficiently.
In case the external circuit has not sufficient
voltage and time to charge the bootstrap
capacitor, extra external circuitry can be used to
ensure the bootstrap voltage in normal operation
region.
The floating driver’s UVLO is not communicated
to the controller.
The DC quiescent current of the floating driver is
about 20μA. Make sure the bleeding current at
SW node is at least higher than this number.
Current Comparator and Current Limit
The power MOSFET current is accurately sensed
via a current sense MOSFET. It is then fed to the
high speed current comparator for the current
mode control purpose. The current comparator
takes this sensed current as one of its inputs.
When the power MOSFET is turned on, the
comparator is first blanked till the end of the turn-
on transition to dodge the noise. Then, the
comparator compares the power switch current
with COMP voltage. When the sensed current is
higher than COMP voltage, the comparator
outputs low, turning off the power MOSFET. The
maximum current of the internal power MOSFET
is internally limited cycle by cycle.
Startup and Shutdown
If both V
IN
and EN are higher than their
appropriate thresholds, the chip starts. The
reference block starts first, generating stable
reference voltage and currents and then the
internal regulator is enabled. The regulator
provides stable supply for the rest circuitries.
While the internal supply rail is up, an internal
timer holds the power MOSFET OFF for about
50usec to blank the startup glitches. When the
internal soft-start block is enabled, it first holds its
SS output low to ensure the rest circuitries are
ready and then slowly ramps up.
Three events shut down the chip: EN low, V
IN
low,
thermal shutdown. In the shutdown procedure,
the signaling path is blocked first to avoid any
fault triggering. COMP voltage and the internal
supply rail are pulled down then. The floating
driver is not subject to this shutdown command
but its charging path is disabled.
MP245136V, 2MHz, 0.6A, STEP-DOWN CONVERTER
MP2451 Rev. 1.33 www.MonolithicPower.com
4/29/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
9
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:
2R1R
2R
VV
OUTFB
+
=
Thus the output voltage is:
2R
)2R1R(
VV
FBOUT
+
=
The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor.
Choose R1 around 124kΩ for optimal transient
response. R2 is then given by:
OUT
R1
R2
V
1
0.8V
=
−
Table 1Resistor Selection vs. Output
Voltage Setting
V
OUT
R1 R2
0.8V 124kΩ (1%) NS
1.2V 124kΩ (1%) 249kΩ (1%)
3.3V 124kΩ (1%) 40.2kΩ (1%)
5V 124kΩ (1%) 23.7kΩ (1%)
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.
Generally, 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 load
current. Also, make sure that the peak inductor
current is below the maximum switch current
limit. The inductance value can be calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×
=
IN
OUT
LS
OUT
V
V
1
ΔIf
V
1L
Where VOUT is the output voltage, VIN is the input
voltage, f
S is the switching frequency, and ΔIL 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:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
××
+=
IN
OUT
S
OUT
LOADLP
V
V
1
1Lf2
V
II
Where I
LOAD is the load current.
Table 2 lists a number of suitable inductors
from various manufacturers. The choice of
which style inductor to use mainly depends on
the price vs. size requirements and any EMI
requirement.
P1-P3P4-P6P7-P9P10-P12P13-P15

MP2451DT-LF-Z

Mfr. #:
Buy MP2451DT-LF-Z
Manufacturer:
Monolithic Power Systems (MPS)
Description:
Switching Voltage Regulators 600mA 36V 2MHz Non-sync Buck
Lifecycle:
New from this manufacturer.
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