TM
MP2104 – 1.7MHz 600mA SYNCHRONOUS STEP-DOWN CONVERTER
IN
+
IAMP
10X
--
CURRENT
SENSE
AMP
PWM
DH
MAIN
SWITCH
(PCH)
PWM
CONTROL
LOGIC
SW
SYNCHRONOUS
RECTIFIER
(NCH)
DL
EN
BIAS
&
VOLTAGE
REFERENCE
ICS
+
SLOPE COMP
EAO
0.6V
FEEDBACK
ERROR EAMP
AMP
--
+
+
PWMCMP
--
OUT
CC
17pF
1.7MHz
OSCILLATOR
OSC
GND
MP2104_BD01
Figure 2—Functional Block Diagram (MP2104DJ-1.5 / MP2104DJ-1.8)
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. The MP2104 switches at a
constant frequency (1.7MHz) 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 either the next cycle starts.
Dropout Operation
The MP2104 allows the main switch to remain on
for more than one switching cycle and increases
the duty cycle while the input voltage is dropping
close to the output voltage. When the duty cycle
reaches 100%, the main switch is held on
continuously to deliver current to the output up to
MP2104 Rev. 1.3
10/4/2005
the PFET current limit. The output voltage then is
the input voltage minus the voltage drop across
the main switch and the inductor.
Short Circuit Protection
The MP2104 has short circuit protection. When
the output is shorted to ground, the oscillator
frequency is reduced to prevent the inductor
current from increasing beyond the PFET current
limit. The PFET current limit is also reduced to
lower the short circuit current. The frequency and
current limit will return to the normal values once
the short circuit condition is removed and the
feedback voltage reaches 0.6V.
Maximum Load current
The MP2104 can operate down to 2.5V input
voltage, however the maximum load current
decreases at lower input due to large IR drop on
the main switch and synchronous rectifier. The
slope compensation signal reduces the peak
inductor current as a function of the duty cycle to
prevent sub-harmonic oscillations at duty cycles
greater than 50%. Conversely the current limit
increases as the duty cycle decreases.
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