CM6900G Datasheet, PDF (8/12 Pages) Champion Microelectronic Corp. – Resonant Controller

VFB +
2.5V
-
CM6900G
Resonant Controller
Vref
V To I
RT
OSC
CT
RSET
PWM Section
Pulse Width Modulator
PWM control is used in voltage mode. In Applicaton1, when FEAO is equal to the external reference voltage at
D_IN+ due to light load, the frequency of the oscillator is fixed at a value determined by the voltage at FEAO, and
the controller enters into PWM mode. In this mode, both frequency modulator and pulse width modulator are active.
In Application 2 ,the offset voltage Vos is externally generated between the V_FB pin and D_IN- through a resistor
network R_f1, R_f2 and R_f3 in order to separate the interaction between FM mode and PWM mode.
Vos = 2.5V×R_f2/R_f3
When the output load is reduced to a point where FEAO is railed to its maximum value of 5V+ VBE, the frequency is
fixed at a value determined by 5V+VBE. Further reduction in load will cause the controller to go into PWM mode
when the voltage at the VFB pin is 2.5V+Vos. During this light load operation, FM error amplifier is in open-loop
mode while PWM is in closed-loop mode. Compensation is accomplished by connecting R and C in series to the
DEAO pin.
In Application 3 , low gain configuration accomplished by connecting R_f4 and R_f5 in the closed loop
configuration . The gain for the PWM is determined by R_f4 and R_f5 where the gain is equal to -R_f5/R_f4. The voltage
of VR_set at which the controller goes into FM and PWM simultaneously is equal to :
VRset = D_IN+ × (1+R_f4/R_f5) - (R_f4/R_f5) × 3 where 3 is the peak voltage of RTCT
Varyin R_set from VR_set to VR_set + 1.75 × R_f4/R_f5 will cause the duty cycle to vary from 50% to 0% while the
frequency will vary propotionally according to 1.75*R_f4/R_f5 where 1.75 is the peak-to-peak voltage of the RTCT
ramp. For proper operation, select R_f5 value so that the current through R_f5 {((D_IN+) - 1.2)/R_f5} should be less
than 5uA.
2007/11/22 Rev1.0
Champion Microelectronic Corporation
Page 8