MP2108DQ MPS [Monolithic Power Systems], MP2108DQ Datasheet - Page 7

• Manufacturer Part Number: MP2108DQ
• Description: 2A, 6V, 740KHz Synchronous Buck Converter
• Manufacturer: MPS [Monolithic Power Systems]

The MP2108 measures the output voltage
through an external resistive voltage divider and
compares it to the internal 0.9V reference to
generate the error voltage at COMP. The
current-mode regulator uses the voltage at
COMP and compares it to the inductor current
to regulate the output voltage. The use of
current-mode regulation improves transient
response and control loop stability.
At the beginning of each cycle, the high-side
N-Channel MOSFET is turned on, forcing the
inductor current to rise. The current at the drain
of
measured and converted to a voltage by the
current sense amplifier.
That voltage is compared to the error voltage at
COMP. When the inductor current rises
sufficiently, the PWM comparator turns off the
high-side switch and turns on the low-side
switch; forcing the inductor current to decrease.
APPLICATION INFORMATION
Internal Low-Dropout Regulator
The internal power to the MP2108 is supplied
from the input voltage (VIN) through an internal
2.4V low-dropout linear regulator, whose output
is VREF. Bypass VREF to SGND with a 10nF
or greater capacitor for proper operation. The
internal regulator can not supply more current
than is required to operate the MP2108.
Therefore, do not apply any external load to
VREF.
MP2108 Rev 1.1
10/2/2006
the
high-side
2.6V to 6V
INPUT
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
MOSFET
10nF
C5
OPEN
Figure 2—Typical Application Circuit
C4
is
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER
2.2nF
internally
© 2006 MPS. All Rights Reserved.
C3
www.MonolithicPower.com
10
10nF
6
8
C6
RUN
SS
COMP
VREF
9
2
VIN
MP2108
SGND
The average inductor current is controlled by
the voltage at COMP, which in turn, is
controlled by the output voltage. Thus the
output voltage controls the inductor current to
satisfy the load.
Since
requires voltage above V
bootstrap capacitor from LX to BST is required
to drive the high-side MOSFET gate. When LX
is driven low (through the low-side MOSFET),
the BST capacitor is internally charged. The
voltage at BST is applied to the high-side
MOSFET gate to turn it on. Voltage is
maintained until the high-side MOSFET is
turned off and the low-side MOSFET is turned
on, and the cycle repeats. Connect a 10nF or
greater capacitor from BST to SW to drive the
high-side MOSFET gate.
Soft-Start
The MP2108 includes a soft-start timer that
slowly ramps the output voltage at startup to
prevent excessive current at the input.
When power is applied to the MP2108, and
RUN is asserted. A 2µA internal current source
charges the external capacitor at SS. As the
capacitor charges, the voltage at SS rises. The
MP2108 internally limits the feedback threshold
voltage at FB to that of the voltage at SS. This
forces the output voltage to rise at the same
5
PGND
BST
1
4
LX
10nF
FB
C7
the
3
7
high-side
OUTPUT
2.5V / 2A
N-Channel
IN
to drive its gate, a
MOSFET
7