lm2612atlx National Semiconductor Corporation, lm2612atlx Datasheet - Page 14

• Manufacturer Part Number: lm2612atlx
• Description: Ic Conv Dc/dc Stp-dwn 10microsmd
• Manufacturer: National Semiconductor Corporation

www.national.com
Frequency Synchronization
(SYNC/MODE Pin)
sharp edged signals from a pulse or function generator can
develop under/overshoot as high as 10V at the end of an
improperly terminated cable.
Overvoltage Protection
The LM2612 has an over-voltage comparator that prevents
the output voltage from rising too high when the device is left
in PWM mode under low-load conditions. Otherwise, the
output voltage could rise out of regulation from the minimum
energy transferred per cycle due to the 200ns minimum
on-time of the PFET switch while in PWM mode. When the
output voltage rises by 50mV over its regulation threshold,
the OVP comparator inhibits PWM operation to skip pulses
until the output voltage returns to the regulation threshold. In
over voltage protection, output voltage and ripple increase
slightly.
Shutdown Mode
Setting the EN input low to SGND places the LM2612 in a
0.02uA (typ) shutdown mode. During shutdown, the PFET
switch, NFET synchronous rectifier, reference, control and
bias of the LM2612 are turned off. Setting EN high to VDD
enables normal operation. While turning on, soft start is
activated.
EN must be set low to turn off the LM2612 during undervolt-
age conditions when the supply is less than the 2.8V mini-
mum operating voltage. The LM2612 is designed for mobile
phones and similar applications where power sequencing is
determined by the system controller and internal UVLO (Un-
der Voltage LockOut) circuitry is unnecessary. The LM2612
has no UVLO circuitry. Although the LM2612 exhibits safe
behavior while enabled at low input voltages, this is not
guaranteed.
Internal Synchronous Rectification
While in PWM mode, the LM2612 uses an internal NFET as
a synchronous rectifier to improve efficiency by reducing
rectifier forward voltage drop and associated power loss. In
general, synchronous rectification provides a significant im-
provement in efficiency whenever the output voltage is rela-
tively low compared to the voltage drop across an ordinary
rectifier diode.
Under moderate and heavy loads, the internal NFET syn-
chronous rectifier is turned on during the inductor current
down-slope in the second part of each cycle. The synchro-
nous rectifier is turned off prior to the next cycle, or when the
inductor current ramps near zero at light loads. The NFET is
designed to conduct through its intrinsic body diode during
transient intervals before it turns on, eliminating the need for
an external diode.
Synchronous rectification is disabled and the NFET con-
ducts through its body diode during the second part of each
cycle while in PFM mode to reduce quiescent current asso-
ciated with the synchronous rectifier’s control circuitry. To
increase efficiency in PFM or PWM conditions, place an
external Schottky diode from PGND to SW.
Current Limiting
A current limit feature allows the LM2612 to protect itself and
external components during overload conditions. Current
(Continued)
14
limiting is implemented using an independent internal com-
parator that trips at current limit of the device. In PWM mode,
cycle-by-cycle current limiting is normally used. If an exces-
sive load pulls the output voltage 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 begin-
ning of the next cycle is inhibited for 2.5µs to force the
instantaneous inductor current to ramp down to a safe value.
PFM mode also uses timed current limit operation. The
synchronous rectifier is off in timed current limit mode. Timed
current limit prevents the loss of current control seen in some
products when the output voltage is pulled low in serious
overload conditions.
Current Limiting and PWM Mode
Transient Response
Considerations
The LM2612 was designed for fast response to moderate
load steps. Harsh transient conditions during loads above
300mA can cause the inductor current to swing up to the
maximum current limit, resulting in PWM mode jitter or insta-
bility from activation of the current limit comparator. To avoid
this jitter or instability, do not power-up or start the LM2612
into a full load (loads near or above 400mA). Do not change
operating modes or output voltages when operating at a full
load. Avoid extremely sharp and wide-ranging load steps to
full load, such as from
Pin Selectable Output Voltage
The LM2612 features pin-selectable output voltage to elimi-
nate the need for external feedback resistors. The output
can be set to 1.05V, 1.3V, 1.5V or 1.8V by configuring the
VID0 and VID1 pins. See Setting the Output Voltage in the
Application Information section for further details.
Soft-Start
The LM2612 has soft start to reduce current inrush during
power-up and startup. This reduces stress on the LM2612
and external components. It also reduces startup transients
on the power source.
Soft start is implemented by ramping up the internal refer-
ence in the LM2612 to gradually increase the output voltage.
The reference ramps up in about 400µs. When powering up
in PWM mode, soft start may take an additional 200us to
allow time for the error amplifier compensation network to
charge.
Thermal Shutdown Protection
The LM2612 has thermal shutdown protection that operates
to protect from short-term misuse and overload conditions.
When the junction temperature exceeds about 150˚C, the
device shuts down, re-starting in soft start after the tempera-
ture drops below 130˚C. Prolonged operation in thermal
overload conditions may damage the device and is consid-
ered bad practice.
<
30mA to
>
350mA.