Texas Instruments LMR61428 Evaluation Module LMR61428XMMEVM/NOPB LMR61428XMMEVM/NOPB Ficha De Dados
Códigos do produto
LMR61428XMMEVM/NOPB
Powering and Loading Considerations
4
Powering and Loading Considerations
Read this entire section prior to attempting to power the evaluation board.
4.1
Quick Start Procedure
Step 1: Set the bench power supply current limit to 3A. Set the power supply voltage to 3.5V. Turn off the
power supply output. Connect the power supply to the LMR61428 demo board. Positive connection to V
power supply output. Connect the power supply to the LMR61428 demo board. Positive connection to V
IN
and negative connection to GND.
Step 2: Connect a load, as high as 0.5A, to the V
OUT
terminal. Positive connection to V
OUT
and negative
connection to GND.
Step 3: Connect the shunt so as to short the pins 1 and 2 of the jumper J1. This sets the bootstrap to V
OUT
Step 4: The EN pin should be left open for normal operation.
Step 5: Turn on the bench power supply with no load applied to the LMR61428. If the shunt for the jumper
J1 was in place, the V
J1 was in place, the V
OUT
would be in regulation at a nominal 5V.
Step 6: Gradually increase the load and V
OUT
should remain in regulation as the load is increased up to
0.5 Amps.
4.2
Shutdown Operation
The EVM includes a pull-up resistor Ren to enable the device. Use the EN post to disable the device by
pulling this node to GND.
pulling this node to GND.
4.3
Bootstrap Operation
The EVM has a jumper installed to select the bootstrap option. The default condition is that the jumper be
set such that the bootstrap voltage is obtained from the output. For more information, see LMR61428
SIMPLE SWITCHER 14Vout, 2.85A Step-Up Voltage Regulator in VSSOP (
set such that the bootstrap voltage is obtained from the output. For more information, see LMR61428
SIMPLE SWITCHER 14Vout, 2.85A Step-Up Voltage Regulator in VSSOP (
).
4.4
Setting the Output Voltage
The output voltage of the step-up regulator can be set between 1.24V and 14V. But because of the gated
oscillator scheme, the maximum possible input to output boost ratio is fixed. For a boost regulator,
oscillator scheme, the maximum possible input to output boost ratio is fixed. For a boost regulator,
V
OUT
/ V
IN
= 1 / [1
−
D]
(1)
The LMR61428 has a fixed duty cycle, D, of 70% typical. Therefore,
V
OUT
/ V
IN
= 1 / 0.3
(2)
This sets the maximum possible boost ratio of V
IN
to V
OUT
to about 3 times. The user can now estimate
what the minimum design inputs should be in order to achieve a desired output, or what the output would
be when a certain minimum input is applied. For example, if the desired V
be when a certain minimum input is applied. For example, if the desired V
OUT
was 14V, then the least V
IN
should be higher than V
OUT
/ 3. If the input voltage fell below this threshold, the output voltage would not
be regulated because of the fixed duty cycle. If the minimum V
IN
was guaranteed at 2V, the max possible
V
OUT
would be V
IN
× 3.
The V
OUT
is set by connecting a feedback resistive divider made of R
fbt
and R
fbb
. The feedback resistor
values are selected as follows:
R
fbb
= R
fbt
/[(V
OUT
/ 1.24)
−
1]
(3)
A value of 150k
Ω
is suggested for R
fbt
. Then, R
fbb
can be selected using
. A 39pF capacitor (C
ff
)
connected across R
fbt
helps in feeding back most of the AC ripple at V
OUT
to the FB pin. This helps reduce
the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because
a set hysteresis of 30mV at the FB pin is used for the gated oscillator control scheme.
a set hysteresis of 30mV at the FB pin is used for the gated oscillator control scheme.
2
AN-2281 LMR61428 Evaluation Module
SNVU193A – October 2012 – Revised April 2013
Copyright © 2012–2013, Texas Instruments Incorporated