LM2619ATLX National Semiconductor, LM2619ATLX Datasheet - Page 11

• Manufacturer Part Number: LM2619ATLX
• Description: Conv DC-DC Single Step Down 2.8V to 5.5V 10-Pin uSMD T/R
• Manufacturer: National Semiconductor
• Type: Step Downr

Specifications of LM2619ATLX

• Package: 10uSMD
• Number Of Outputs: 1
• Minimum Input Voltage: 2.8 V
• Maximum Input Voltage: 5.5 V
• Switching Frequency: 500 to 1000 KHz
• Operating Supply Voltage: 2.8 to 5.5 V
• Maximum Output Current: 0.5 A
• Output Type: Adjustable
• Output Voltage: 1.5 to 3.6 V
• Switching Regulator: Yes
• Efficiency: 96(Typ) %
• Operating Temperature: -25 to 125 °C

Device Information
less than the 2.8V minimum operating voltage. The LM2619
is designed for compact portable applications, such as mo-
bile phones. In such applications, the system controller de-
termines power supply sequencing. Although the LM2619 is
typically well behaved at low input voltages, this is not guar-
anteed.
INTERNAL SYNCHRONOUS RECTIFICATION
While in PWM mode, the LM2619 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever
the output voltage is relatively low compared to the voltage
drop across an ordinary rectifier diode.
The internal NFET synchronous rectifier is turned on during
the inductor current down slope during the second part of
each cycle. The synchronous rectifier is turned off prior to the
next cycle, or when the inductor current ramps to zero at light
loads. The NFET is designed to conduct through its intrinsic
body diode during transient intervals before it turns on, elimi-
nating the need for an external diode.
CURRENT LIMITING
A current limit feature allows the LM2619 to protect itself and
external components during overload conditions. In PWM
mode cycle-by-cycle current limit is normally used. If an
excessive load pulls the voltage at the feedback pin down to
approximately 0.7V, then the device switches to a timed
current limit mode. In timed current limit mode the internal
P-FET switch is turned off after the current comparator trips
and the beginning of the next cycle is inhibited for 2.5µs to
force the instantaneous inductor current to ramp down to a
safe value. Timed current limit mode prevents the loss of
current control seen in some products when the voltage at
the feedback pin is pulled low in serious overload conditions.
DROPOUT CONSIDERATIONS
The LM2619 can be used to provide fixed output voltages by
using external feedback resistors. The output voltage can be
set from 1.5V to 3.6V. The internal reference voltage for the
error amplifier is 1.5V. In cases where the output voltage is
set higher than 2.5V, the part will go into dropout or 100%
duty cycle when the input voltage gets close to the set output
voltage. Near dropout the on time of the P-FET may exceed
one PWM clock cycle and cause higher ripple on the output
for load currents greater than 450mA. This increased ripple
will exist for a narrow range of input voltages close to the
100% duty cycle and once the input voltage goes down
further the P-FET will be fully on. See Setting the Output
Voltage in the Application Information section for further
details.
In dropout conditions the output voltage is V
R
and R
DSON (P)
DSON (P)
) where Rdc is the series resistance of the inductor
is the on resistance of the PFET.
(Continued)
IN
− I
OUT
(Rdc +
11
SOFT-START
The LM2619 has soft start to reduce current inrush during
power-up and startup. This reduces stress on the LM2619
and external components. It also reduces startup transients
on the power source. Soft start is implemented by ramping
up the reference input to the error amplifier of the LM2619 to
gradually increase the output voltage.
THERMAL SHUTDOWN PROTECTION
The LM2619 has a thermal shutdown protection function to
protect itself from short-term misuse and overload condi-
tions. When the junction temperature exceeds 150˚C the
device turns off the output stage and when the temperature
drops below 130˚C it initiates a soft start cycle. Prolonged
operation in thermal shutdown conditions may damage the
device and is considered bad practice.
Load Transient Response
Line Transient Response
(Circuit in Figure 3)
(Circuit in Figure 3)
FIGURE 8.
FIGURE 9.
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