Datasheet

5
DCMHI DCM
DCMHI DCM
R R
V 2 10
R R
-
´
D = ´
+
PWM
DCM Threshold
+ Hysteresis
CS
E
F
PWM
DCM Threshold
+ Hysteresis
CS
E
F
UCC28950
www.ti.com SLUSA16B –MARCH 2010–REVISED OCTOBER 2011
There is a nominal 20-µA switched current source used to create hysteresis. The current source is active only
when the system is in DCM Mode. Otherwise, it is inactive and does not affect the node voltage. Therefore, when
being in DCM region, the DCM threshold is the voltage divider plus ΔV explained in Equation 14 below. When
being in CCM region, the threshold is the voltage set by the resistor divider. When CS pin reaches the threshold
set on the DCM pin, the system waits to see two consecutive falling edge PWM cycles before switching from
CCM to DCM and vice-versa. The magnitude of the hysteresis is a function of the external resistor divider
impedance. The hysteresis can be calculated using the following Equation 14:
(14)
Figure 16. Moving from DCM to CCM Mode
Figure 17. Moving from CCM to DCM Mode
DCM must be used in order to prevent reverse current in the output inductor which could cause the synchronous
FETS to fail.
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