CM3706 Datasheet, PDF (5/11 Pages) Champion Microelectronic Corp. – 1.5MHz, 600mA Synchronous Step-Down Regulator

CM3706
1.5MHz, 600mA Synchronous
Step-Down Regulator
FUNCTIONAL DESCRIPTION
Operation
CM3706 is a monolithic switching mode Step-Down DC-DC
converter. It utilizes internal MOSFETs to achieve high
efficiency and can generate very low output voltage by using
internal reference at 0.6V. It operates at a fixed switching
frequency, and uses the slope compensated current mode
architecture. This Step-Down DC-DC Converter supplies
600mA output current at VIN = 3V with input voltage range
from 2.5V to 5.5V.
Idle Mode Operation
At very light loads, the CM3706 automatically enters Idle
Mode. In the Idle Mode, the inductor current may reach zero
or reverse on each pulse. The PWM control loop will
automatically skip pulses to
maintain output regulation. The bottom MOSFET is turned
off by the current reversal comparator, IZERO, and the switch
voltage will ring. This is discontinuous mode operation, and
is normal behavior for the switching regulator.
Current Mode PWM Control
Slope compensated current mode PWM control provides
stable switching and cycle-by-cycle current limit for excellent
load and line responses and protection of the internal main
switch (P-Ch MOSFET) and synchronous rectifier (N-CH
MOSFET). During normal operation, the internal P-Ch
MOSFET 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. The current comparator, ICOMP,
limits
the peak inductor current. When the main switch is off, the
synchronous rectifier will be turned on immediately and stay
on until either the inductor current starts to reverse, as
indicated by the current reversal comparator, IZERO, or the
beginning of the next clock cycle. The OVDET comparator
controls output transient overshoots by turning the main
switch off and keeping it off until the fault is no longer present.
Maximum Load Current
The CM3706 will operate with input supply voltage as
low as 2.5V, 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.
Note 5: The duty cycle D of a step-down converter is defined
as:
D
= T_ON
×
f _OSC
×100%
≈ ^{V OUT}
V_IN
×100%
where T_ON is the main switch on time, and f_OSC is the oscillator
frequency (1.5Mhz).
Dropout Operation
When the input voltage decreases toward the value of the
output voltage, the CM3706 allows the main switch to remain
on for more than one switching cycle and increases the duty
cycle (Note 5) until it reaches 100%. The output voltage then
is the input voltage
minus the voltage drop across the main switch and the
inductor. At low input supply voltage, the RDS(ON) of the
P-Channel MOSFET increases, and the efficiency of the
converter decreases. Caution must be exercised to ensure
the heat dissipated not to exceed the maximum junction
temperature of the IC.
2006/12/20Rev. 1.6
Champion Microelectronic Corporation
Page 5