TPS62020_16 Datasheet, PDF (11/28 Pages) Texas Instruments – 600 mA/1.25 MHz HIGH-EFFICIENCY STEP-DOWN CONVERTER

www.ti.com
DETAILED DESCRIPTION (continued)
TPS62020
TPS62021
TPS62026
SLVS076C – JUNE 2003 – REVISED DECEMBER 2004
1.6%
0.8%
V_O
PFM Mode at Light Load
Comp High
Comp Low
Comp Low 2
PWM Mode at Medium to Full Load
Figure 14. Power Save Mode Thresholds and Dynamic Voltage Positioning
The converter enters the fixed frequency PWM mode as soon as the output voltage falls below the comp low 2
threshold.
DYNAMIC VOLTAGE POSITIONING
As described in the power save mode operation sections before and as detailed in Figure 14 the output voltage
is typically 0.8% (i.e., 1% on average) above the nominal output voltage at light load currents, as the device is in
power save mode. This gives additional headroom for the voltage drop during a load transient from light load to
full load. In the other direction during a load transient from full load to light load the voltage overshoot is also
minimized by turning on the N-Channel rectifier switch to pull the output voltage actively down.
MODE (AUTOMATIC PWM/PFM OPERATION AND FORCED PWM OPERATION)
Connecting the MODE pin of the TPS6202x to GND enables the automatic PWM and power save mode
operation. The converter operates in fixed frequency PWM mode at moderate to heavy loads and in the PFM
mode during light loads, maintaining high efficiency over a wide load current range.
Pulling the TPS6202x MODE pin high forces the converter to operate constantly in the PWM mode even at light
load currents. The advantage is the converter operates with a fixed switching frequency that allows simple
filtering of the switching frequency for noise sensitive applications. In this mode, the efficiency is lower compared
to the power save mode during light loads (see Figure 1 to Figure 3). For additional flexibility it is possible to
switch from power save mode to forced PWM mode during operation. This allows efficient power management
by adjusting the operation of the TPS6202x to the specific system requirements.
The difference between the TPS6202x and the TPS62021 is the logic level of the MODE pin. The TPS62021 has
an active-low MODE pin. Pulling the TPS62021 MODE pin high enables the automatic PWM and Power Save
Mode.
100% DUTY CYCLE LOW DROPOUT OPERATION
The TPS6202x offers a low input to output voltage difference while still maintaining regulation with the use of the
100% duty cycle mode. In this mode, the P-Channel switch is constantly turned on. This is particularly useful in
battery powered applications to achieve longest operation time by taking full advantage of the whole battery
voltage range. i.e. The minimum input voltage to maintain regulation depends on the load current and output
voltage and can be calculated as:
ǒ Ǔ VI min + VO max ) IO max rDS(on) max ) RL
(2)
with:
•
•
I_O(max) = maximum output current plus inductor ripple current
r_DS(on)max = maximum P-channel switch t_DS(on).
11