CM3706A 查看數據表（PDF） - Champion Microelectronic

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CM3706A

1.5MHz, 600mA Synchronous Step-Down Regulator

Champion Microelectronic 

CM3706A

1.5MHz, 600mA Synchronous Step-Down Regulator

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.

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. 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, limitsthe 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.

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 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.

Maximum Load Current

The CM3706A 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

= TON

×

f OSC

×100%

≈ V OUT

VIN

×100%

where TON is the main switch on time, and fOSC is the oscillator

frequency (1.5Mhz).

2008/03/07 Rev. 1.0

Champion Microelectronic Corporation

Page 6

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