lm21215mhx National Semiconductor Corporation, lm21215mhx Datasheet - Page 10

• Manufacturer Part Number: lm21215mhx
• Description: 15a High Efficiency Synchronous Buck Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
GENERAL
The LM21215 switching regulator features all of the functions
necessary to implement an efficient low voltage buck regula-
tor using a minimum number of external components. This
easy to use regulator features two integrated switches and is
capable of supplying up to 15A of continuous output current.
The regulator utilizes voltage mode control with trailing edge
modulation to optimize stability and transient response over
the entire output voltage range. The device can operate at
high switching frequency allowing use of a small inductor
while still achieving high efficiency. The precision internal
voltage reference allows the output to be set as low as 0.6V.
Fault protection features include: current limiting, thermal
shutdown, over voltage protection, and shutdown capability.
The device is available in the eTSSOP-20 package featuring
an exposed pad to aid thermal dissipation. The LM21215 can
be used in numerous applications to efficiently step-down
from a 5V or 3.3V bus.
PRECISION ENABLE
The enable (EN) pin allows the output of the device to be en-
abled or disabled with an external control signal. This pin is a
precision analog input that enables the device when the volt-
age exceeds 1.35V (typical). The EN pin has 110 mV of
hysteresis and will disable the output when the enable voltage
falls below 1.24V (typical). If the EN pin is not used, it can be
left open, and will be pulled high by an internal 2 µA current
source. Since the enable pin has a precise turn-on threshold
it can be used along with an external resistor divider network
from VIN to configure the device to turn-on at a precise input
voltage.
10
UVLO
The LM21215 has a built-in under-voltage lockout protection
circuit that keeps the device from switching until the input
voltage reaches 2.7V (typical). The UVLO threshold has 200
mV of hysteresis that keeps the device from responding to
power-on glitches during start up. If desired the turn-on point
of the supply can be changed by using the precision enable
pin and a resistor divider network connected to VIN as shown
in
CURRENT LIMIT
The LM21215 has programmable current limit protection to
avoid dangerous current levels through the power FETs and
inductor. A current limit condition is met when the current
through the high side FET exceeds the rising current limit level
(I
off the high side FET and turning on the low side FET. This
forces a negative voltage on the inductor, thereby causing the
inductor current to decrease. The high side FET will not con-
duct again until the lower current limit level (I
the low side FET. At this point, the device will resume normal
switching.
A current limit condition will cause the internal soft-start volt-
age to ramp downward. After the internal soft-start ramps
below the Feedback (FB) pin voltage, (nominally 0.6 V), FB
will begin to ramp downward, as well. This voltage foldback
will limit the power consumption in the device, thereby pro-
tecting the device from continuously supplying power to the
load under a condition that does not fall within the device
SOA. After the current limit condition is cleared, the internal
soft-start voltage will ramp up again.
limit behavior with V
CLR
Figure 5
). The control circuitry will respond to this event by turning
in the design guide.
SS
, V
FB
, V
OUT
and V
Figure 1
SW
.
CLF
shows current
) is sensed on