TPS53219A_15 Datasheet, PDF (16/35 Pages) Texas Instruments – Single Synchronous Step-Down Controller

TPS53219A
SLUSAU4A – DECEMBER 2011 – REVISED DECEMBER 2015
www.ti.com
7.4 Device Functional Modes
7.4.1 Light Load Condition in Auto-Skip Operation
While the MODE pin is pulled low through R_MODE, TPS53219A automatically reduces the switching frequency at
light load conditions to maintain high efficiency. Detailed operation is described as follows. As the output current
decreases from heavy load condition, the inductor current is also reduced and eventually comes to the point that
its rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous
conduction modes. The synchronous MOSFET is turned off when this zero inductor current is detected. As the
load current further decreases, the converter runs into discontinuous conduction mode (DCM). The ON-time is
kept almost the same as it was in the continuous conduction mode so that it takes longer time to discharge the
output capacitor with smaller load current to the level of the reference voltage. The transition point to the light
load operation I_O(LL) (that is, the threshold between continuous and discontinuous conduction mode) can be
calculated as shown in Equation 8.
( ) 1
I_OUT(LL) = _{2 ´ L ´ fSW} ´
V_IN - V_OUT
V_IN
´ V_OUT
where
• ƒ_SW is the PWM switching frequency
(8)
Switching frequency versus output current in the light load condition is a function of L, V_IN and V_OUT, but it
decreases almost proportionally to the output current from the IO_(LL) given in Equation 8. For example, it is 60
kHz at IO_(LL)/5 if the frequency setting is 300 kHz.
7.4.2 Forced Continuous Conduction Mode
When the MODE pin is tied to PGOOD through a resistor, the controller keeps continuous conduction mode
(CCM) in light load condition. In this mode, switching frequency is kept almost constant over the entire load
range which is suitable for applications need tight control of the switching frequency at a cost of lower efficiency.
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