CM2500 Datasheet, PDF (6/11 Pages) Champion Microelectronic Corp. – WIDE INPUT 2A STEP DOWN CONVERTER

CM2500
WIDE INPUT 2A STEP DOWN CONVERTER
FUNCTIONAL DESCRIPTION
As seen in Figure 2, Functional Block Diagram, the CM2500
is a current mode pulse width modulation (PWM) converter.
The converter operates as follows : A switching cycle starts
when the rising edge of the Oscillator clock output causes
the High- Side Power Switch to turn on and the Low-Side
Power Switch to turn off. With the SW side of the inductor
now connected to IN, the inductor current ramps up to store
energy in the magnetic field. The inductor current level is
measured by the Current Sense Amplifier and added to the
Oscillator ramp signal. If the resulting summation is higher
than the COMP voltage, the output of the PWM Comparator
goes high. When this happens or when Oscillator clock
output goes low, the High-Side Power Switch turns off and
the Low-Side Power Switch turns on. At this point, the SW
side of the inductor swings to a diode voltage below ground,
causing the inductor current to decrease and magnetic
energy to be transferred to output. This state continues until
the cycle starts again. The High-Side Power Switch is driven
by logic using BS as the positive rail. This pin is charged to
VSW + 6V when the Low-Side Power Switch turns on.
The COMP voltage is the integration of the error between FB
input and the internal 1.28V reference. If FB is lower than the
reference voltage, COMP tends to go higher to increase
current to the output. Current limit happens when COMP
reaches its maximum clamp value of 2.55V.
The Oscillator normally switches at 380kHz. However, if FB
voltage is less than 0.7V, then the switching frequency
decreases until it reaches a minimum of 50kHz at VFB =
0.5V.
SHUTDOWN CONTROL
The CM2500 has an enable input EN for turning the IC on or
off. When EN is less than 1.8V, the IC is in 8μA low current
shutdown mode and output is discharged through the Low-
Side Power Switch. When EN is higher than 1.85V, the IC is
in normal operation mode. EN is internally pulled up with a
2.5μA current source and can be left unconnected for
always-on operation. Note that EN is a low voltage input with
a maximum voltage of 6V; it should never be directly
connected to IN.
THERMAL SHUTDOWN
The CM2500 automatically turns off when its junction
temperature exceeds 170°C.
APPLICATION INFORMATION
OUTPUT VOLTAGE SETTING
Figure 4 shows the connections for setting the output voltage.
Select the proper ratio of the two feedback resistors RFB1 and
RFB2 based on the output voltage. Typically, use RFB2 ≈
10kΩ and determine RFB1 from the following equation:
INDUCTOR SELECTION
The inductor maintains a continuous current to the output load.
This inductor current has a ripple that is dependent on the
inductance value: higher inductance reduces the peak-to-peak
ripple current. The trade off for high inductance value is the
increase in inductor core size and series resistance, and the
reduction in current handling capability. In general, select an
inductance value L based on the ripple current requirement:
where VIN is the input voltage, VOUT is the output voltage,
fSW is the switching frequency, IOUTMAX is the maximum
output current, and KRIPPLE is the ripple factor. Typically,
choose KRIPPLE = 30% to correspond to the peak-to-peak
ripple current being 30% of the maximum output current. With
this inductor value, the peak inductor current is IOUT • (1 +
KRIPPLE / 2). Make sure that this peak inductor current is less
that the 3A current limit. Finally, select the inductor core size so
that it does not saturate at 3A. Typical inductor values for
various output voltages are shown in Table 1.
Table 1. Typical Inductor Values
VOUT
1.5V 1.8V 2.5V 3.3V 5V
L
6.8μH 6.8μH 10μH 15μH 22μH
2008/04/15 Rev1.0
Champion Microelectronic Corporation
Page 6