SC4806MLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC4806MLTRT Datasheet PDF : 22 Pages
| |||
SC4806
POWER MANAGEMENT
Application Information (Cont.)
OPERATION MODE
SC4806 can be configured in either voltage mode or cur-
rent mode. In voltage mode, a ramp is externally gener-
ated by RC network. The R can be connected to Vref or
other fixed voltage source as shown below. By compar-
ing control signal to the ramp, PWM duty cycle is derived.
SC4806 Voltage mode (Non-Feed Forward)
Vref or other fixed Voltage source
R1
Ramp
C1
In current mode control, the ramp voltage is not derived
artificially from a ramp generator. It is instead provided
from a power converter inductor current by a current
sensing transformer or resistor. Thus a second, inner
control loop is formed by comparing the inductor current
ramp to control voltage from outer voltage loop. Now
the control voltage programs the inductor current via the
inner loop and no longer controls the duty cycle directly.
The current mode control corrects most of problems with
direct duty cycle control in voltage mode. The chief ad-
vantage of the methods its inherent feed-forward char-
acteristics and simplified loop dynamics. An added ben-
efits is the reduction or elimination of transformer satu-
ration problems in full-bridge or push-pull isolated con-
verters. The current mode configuration with SC4806 is
as shown below:
SC4806 100% Current mode (No Slope Compensation)
Isense
R3
ILIM
C2
Ramp
R3
Isense
ILIM
Voltage mode with feed-forward operation is imple-
mented if the R is connected to input voltage as shown
below. With this implementation, the ramp amplitude
varies directly with input voltage. If control signal to FB is
constant, the duty cycle varies inversely with input volt-
age. Thus the volt-second product, Vin*D, remains con-
stant without any control change. Open loop line regula-
tion better than direct duty cycle control as shown above.
Good dynamic response is achieved with less closed loop
gain required.
C2
The current mode control ling the peak inductor current
results in circuit instability whenever the steady state duty
cycle is greater than 0.5. An artificial slope has to be
added to avoid such problem. Power transformer mag-
netizing current riding on the reflected inductor current
acts to provide some slope compensation, but the
amount is rather variable and indeterminate. The cur-
rent mode with slope compensation is as shown below:
SC4806 Voltage mode (Feed Forward)
Vin
SC4806 Current mode (With Slope Compensation)
Vref or other fixed Voltage source
R1
Ramp
C1
Isense
R3
ILIM
C2
RC
Ramp
ILIM
Isense
2006 Semtech Corp.
11
www.semtech.com
Share Link: 