Test Summary
4.3
Test Procedure Using a Single Cell Li-Ion Battery
1. Make sure that the EVM is set up according to
and
, and that the power supply is
preset to 3.3 VDC at ~150 mA current limit.
2. Turn on the input supply and verify the input voltage is ~3.3 VDC (DMM#1) and the output voltage is at
~2.1 VDC (DMM#2).
3. Look at CH1 and CH2 and verify that the duty cycle is near 70% and the ripple is less than 10-mV
ripple; see
for typical waveforms.
4. Vary the load between 0 and 100 mA (1 k
Ω
to 21
Ω
). Observe the change in the switching waveform
from PFM with discontinuous ringing to PWM mode. It may be necessary to change the time scale on
the scope to 1
µ
s/div for light loads. See
and
for various loads. Set the load back to
approximately 21
Ω
.
5. Vary the input voltage from 3.3 VDC to 3.9 VDC and back to 2.4 VDC to see the change in duty cycle.
6. Reduce the input voltage from 2.4 VDC to 1.9 VDC and verify that the switcher automatically goes into
bypass mode, disabling the switcher and turning on the internal bypass FET. The output should be the
input voltage minus the IR drop across the pass FET (~2
Ω
). The STAT pin should go to the
high-impedance state and be pulled to the output voltage.
was captured by moving CH2 to the
J7-STAT relative to ground, 1 V/div, dc-coupled, setting the time scale to 50
µ
s/div (a slower time scale
may be required, depending on power-supply decay), single-sequence trigger on CH2, and removing
input power. The phase node is shown going into PFM, then disabling switching, and then indicating
bypass mode by the STAT pin being pulled up to the output voltage (~2.2 VDC).
CAUTION
The following step disables the buck converter and switches the input voltage
to the output. Make sure the maximum system input voltage is not exceeded by
this step or the later steps that place the JP1 shunt in the bypass position.
7. Move the shunt on jumper JP1 from the ON position to the Bypass position. Notice that the switcher is
disabled and the input voltage is switched to the output via the bypass switch. The output should be
the input voltage minus the IR drop across the pass FET (~2 ohms). The STAT pin should go to the
high-impedance state and be pulled to the output voltage. Move the shunt on jumper JP1 to the ON
position.
8. For steps 9 through 13, one can view the figure and determine how scope was set up.
9. See
for the transition from converter switch mode to bypass mode by pulling the ON/BYP pin
low.
10. Remove the bypass jumper to see transition from bypass mode to converter switch mode; see
.
11. See
for typical hot-plug power up.
12. See
for the transient output load step from 50 mA to 100 mA.
13. See
for the transient output load step from 100 mA to 50 mA.
4
TPS62730 Stepdown Converter With Bypass Mode for Ultralow-Power Wireless
SLVU455
–
April 2011
Applications
Copyright
©
2011, Texas Instruments Incorporated
