TPS53353_15 Datasheet, PDF (18/37 Pages) Texas Instruments – TPS53353 High-Efficiency 20-A Synchronous Buck Converter With Eco-mode™

TPS53353
SLUSAK2B – AUGUST 2011 – REVISED FEBRUARY 2015
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Device Functional Modes (continued)
According to Equation 6, the loop stability of D-CAP^TM mode modulator is mainly determined by the capacitor's
chemistry. For example, specialty polymer capacitors (SP-CAP) have an output capacitance in the order of
several 100 µF and ESR in range of 10 mΩ. These makes ƒ₀ on the order of 100 kHz or less, creating a stable
loop. However, ceramic capacitors have an ƒ₀ at more than 700 kHz, and need special care when used with this
modulator. An application circuit for ceramic capacitor is described in the External Component Selection Using
All Ceramic Output Capacitors section.
7.4.2 Enable, Soft Start, and Mode Selection
When the EN pin voltage rises above the enable threshold voltage (typically 1.2 V), the controller enters its start-
up sequence. The internal LDO regulator starts immediately and regulates to 5 V at the VREG pin. The controller
then uses the first 250 μs to calibrate the switching frequency setting resistance attached to the RF pin and
stores the switching frequency code in internal registers. During this period, the MODE pin also senses the
resistance attached to this pin and determines the soft-start time. Switching is inhibited during this phase. In the
second phase, an internal DAC starts ramping up the reference voltage from 0 V to 0.6 V. Depending on the
MODE pin setting, the ramping up time varies from 0.7 ms to 5.6 ms. Smooth and constant ramp-up of the
output voltage is maintained during start-up regardless of load current.
Table 3. Soft-Start and MODE Settings
MODE SELECTION
ACTION
Auto-skip
Pulldown to GND
Forced CCM⁽¹⁾
Connect to PGOOD
SOFT-START TIME (ms)
0.7
1.4
2.8
5.6
0.7
1.4
2.8
5.6
R_MODE (kΩ)
39
100
200
475
39
100
200
475
(1) Device enters FCCM after the PGOOD pin goes high when MODE is connected to PGOOD through
the resistor R_MODE.
After soft-start begins, the MODE pin becomes the input of an internal comparator which determines auto-skip or
FCCM mode operation. If MODE voltage is higher than 1.3 V, the converter enters into FCCM mode. Otherwise
it will be in auto-skip mode at light load condition. Typically, when FCCM mode is selected, the MODE pin is
connected to PGOOD through the R_MODE resistor, so that before PGOOD goes high the converter remains in
auto-skip mode.
7.4.3 Auto-Skip Eco-mode™ Light Load Operation
While the MODE pin is pulled low via R_MODE, TPS53353 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 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_OUT(LL) (that is, the threshold between continuous and discontinuous conduction mode) can be
calculated as shown in Equation 7.
( ) 1
I_OUT(LL) = _{2 ´ L ´ fSW} ´
V_IN - V_OUT
V_IN
´ V_OUT
where
• ƒ_SW is the PWM switching frequency
(7)
18
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