TM
MP2109 – DUAL 1.2MHz 800mA SYNCHRONOUS STEP-DOWN CONVERTER
OPERATION
IN1/
IN2
BIAS
&
VOLTAGE
REFERENCE
ICS
+
SLOPE COMP
EAO
+
IAMP
10X
--
CURRENT
SENSE
AMP
PWM
DH
MAIN
SWITCH
(PCH)
PWM
CONTROL
LOGIC
SW1/
SW2
SYNCHRONOUS
RECTIFIER
(NCH)
DL
EN1/
EN2
0.6V
FEEDBACK
ERROR\ EAMP
AMP
--
+
+
PWMCMP
--
FB1/
FB2
CC
17pF
1.0MHz
OSCILLATOR
OSC
GND1/
GND2
MP2109_BD02
Figure 1—Functional Block Diagram (Diagram represents �½ of the MP2109)
The MP2109 has dual independent constant
frequency current mode PWM step-down
converters. The MP2109 is optimized for low
voltage, Li-Ion battery powered applications
where high efficiency and small size are critical.
The MP2109 uses external resistor dividers to
set two output voltages independently from
0.6V to 6V. The device integrates both main
switches and synchronous rectifiers, which
provides high efficiency and eliminates the
need for an external Schottky diode. The
MP2109 can achieve 100% duty cycle. The
duty cycle D of each step-down converter is
defined as:
D
=
T
ON
×
f
OSC
×
100%
=
V
OUT
×
100%
V
IN
Where T
ON
is the main switch on time, f
OSC
is
the oscillator frequency (1.2MHz), V
OUT
is the
output voltage and V
IN
is the input voltage.
Current Mode PWM Control
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle
current limit for superior load and line response
and protection of the internal main switch and
synchronous rectifier. Each channel switches at
a constant frequency (1.2MHz) and regulates
the output voltage. During each cycle the PWM
comparator modulates the power transferred to
the load by changing the inductor peak current
based on the feedback error voltage. During
normal operation, the main switch is turned on
for a certain time to ramp the inductor current at
each rising edge of the internal oscillator, and
switched off when the peak inductor current is
above the error voltage. When the main switch
is off, the synchronous rectifier will be turned on
immediately and stay on until the next cycle
starts.
MP2109 Rev.1.3
2/21/2006
www.MonolithicPower.com
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© 2006 MPS. All Rights Reserved.
5
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