CM3706AGIM25 CHAMP [Champion Microelectronic Corp.], CM3706AGIM25 Datasheet - Page 6

• Manufacturer Part Number: CM3706AGIM25
• Description: 1.5MHz, 600mA Synchronous Step-Down Regulator
• Manufacturer: CHAMP [Champion Microelectronic Corp.]

FUNCTIONAL DESCRIPTION
Operation
CM3706A 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 6.5V.
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, I
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, I
next clock cycle.
transient overshoots by turning the main switch off and
keeping it off until the fault is no longer present.
Dropout Operation
When the input voltage decreases toward the value of the
output voltage, the CM3706A 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 voltageminus the voltage drop across
the main switch and the inductor. At low input supply voltage,
the R
efficiency of the converter decreases.
exercised to ensure the heat dissipated not to exceed the
maximum junction temperature of the IC.
2008/03/07
DS
(ON) of the P-Channel MOSFET increases, and the
Rev. 1.0
The OVDET comparator controls output
ZERO
, or the beginning of the
COMP
Champion Microelectronic Corporation
Caution must be
1.5MHz, 600mA Synchronous Step-Down Regulator
, limitsthe peak
Idle Mode Operation
At very light loads, the CM3706A automatically enters Idle
Mode. In the Idle Mode, the inductor current may reach zero
or reverse on each pulse.
automatically skip pulses to
maintain output regulation. The bottom MOSFET is turned
off by the current reversal comparator, I
voltage will ring. This is discontinuous mode operation, and
is normal behavior for the switching regulator.
Maximum Load Current
The
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.
compensation signal reduces the peak inductor current
as a function of the duty cycle to prevent sub-harmonic
oscillations
Conversely the current limit increases as the duty cycle
decreases.
Note 5: The duty cycle D of a step-down converter is defined
as:
where T
frequency (1.5Mhz).
D
=
CM3706A
T
ON
ON
×
is the main switch on time, and f
f
OSC
at
will operate with input supply voltage as
×
100
duty
%
≈
cycles
V
V
The PWM control loop will
OUT
IN
×
greater
100
CM3706A
ZERO
%
OSC
, and the switch
is the oscillator
Page 6
than
The slope
50%.