Datasheet

LMR61428
www.ti.com SNVS815A –JUNE 2012–REVISED APRIL 2013
LOW VOLTAGE START-UP
The LMR61428 can start-up from input voltages as low as 1.1V. On start-up, the control circuitry switches the N-
channel MOSFET continuously at 70% duty cycle until the output voltage reaches 2.5V. After this output voltage
is reached, the normal step-up regulator feedback and gated oscillator control scheme take over. Once the
device is in regulation it can operate down to a 0.65V input, since the internal power for the IC can be boot-
strapped from the output using the V
DD
pin.
SHUTDOWN
The LMR61428 features a shutdown mode that reduces the quiescent current to less than an ensured 2.5µA
over temperature. This extends the life of the battery in battery powered applications. During shutdown, all
feedback and control circuitry is turned off. The regulator's output voltage drops to one diode drop below the
input voltage. Entry into the shutdown mode is controlled by the active-low logic input pin EN (Pin 2). When the
logic input to this pin pulled below 0.15V
DD
, the device goes into shutdown mode. The logic input to this pin
should be above 0.7V
DD
for the device to work in normal step-up mode.
OUTPUT VOLTAGE RIPPLE FREQUENCY
A major component of the output voltage ripple is due to the hysteresis used in the gated oscillator control
scheme. The frequency of this voltage ripple is proportional to the load current. The frequency of this ripple does
not necessitate the use of larger inductors and capacitors. The size of these components is determined by the
switching frequency of the oscillator which can be set upto 2MHz using an external resistor.
INTERNAL CURRENT LIMIT AND THERMAL PROTECTION
An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85A typical,
approximately 4A maximum) so as not to come into effect during normal operating conditions. An internal thermal
protection circuitry disables the MOSFET power switch when the junction temperature (T
J
) exceeds about 160°C.
The switch is re-enabled when T
J
drops below approximately 135°C.
Design Procedure
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,
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. E.g. 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 ensured 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
F1
and R
F2
. The feedback resistor values
are selected as follows:
R
F2
= R
F1
/[(V
OUT
/ 1.24) −1] (3)
A value of 150kΩ is suggested for R
F1
. Then, R
F2
can be selected using the above equation. A 39pF capacitor
(C
ff
) connected across R
F1
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.
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