lm22676mrx-adj National Semiconductor Corporation, lm22676mrx-adj Datasheet - Page 8

• Manufacturer Part Number: lm22676mrx-adj
• Description: 3a Simple Switcher Step-down Voltage Regulator With Precision Enable
• Manufacturer: National Semiconductor Corporation

www.national.com
Detailed Operating Description
The LM22676 switching regulator features all of the functions
necessary to implement an efficient high voltage buck regu-
lator using a minimum of external components. This easy to
use regulator integrates a 42V N-Channel switch with an out-
put current capability of 3A. The regulator control method is
based on voltage mode control with input voltage feed for-
ward. The loop compensation is integrated into the LM22676
so that no external compensation components need to be se-
lected or utilized. Voltage mode control offers short minimum
on-times allowing short duty-cycles necessary in high input
voltage applications. The operating frequency is fixed at
500kHz to allow for small external components while avoiding
excessive switching losses. The output voltage can be set as
low as 1.285V with the -ADJ device. Fault protection features
include current limiting, thermal shutdown and remote shut-
down capability. The device is available in the TO-263 THIN
and PSOP-8 packages featuring an exposed pad to aid ther-
mal dissipation.
The functional block diagram with typical application of the
LM22676 are shown in Figure 1.
The internal compensation of the -ADJ option of the LM22676
is optimized for output voltages up to 5V. If an output voltage
of 5V or higher is needed, the -5.0 fixed output voltage option
with an additional external resistive feedback voltage divider
may also be used.
Precision Enable
The precision enable pin (EN) can be used to shut down the
power supply. Connecting this pin to ground or to a voltage
less than typical 1.6V will completely turn off the regulator.
The current drain from the input supply when off is typically
25 µA with 12V input voltage. The power consumed during
this off state is mostly defined by an internal 2 MΩ resistor to
VIN. The enable pin has an internal pull-up current source of
approximately 6 µA. When driving the enable pin, the high
voltage level for the on condition should not exceed the 6V
absolute maximum limit. When enable control is not required,
the EN pin should be left floating. The precision feature en-
ables simple sequencing of multiple power supplies with a
resistor divider from another power supply.
Maximum Duty-Cycle / Dropout
Voltage
The typical maximum duty-cycle is 85% at 500 kHz switching
frequency. This corresponds to a typical minimum off-time of
300 ns. When operating at switching frequencies higher than
500 kHz, the 300 ns minimum off-time results in a lower max-
imum duty-cycle limit than 85%. This forced off-time is impor-
tant to provide enough time for the Cboot capacitor to charge
during each cycle.
The lowest input voltage required to maintain operation is:
Where V
Schottky diode and V
power N-FET of the LM22676. The R
specified in the electrical characteristics section of this
datasheet to calculate V
the switching frequency.
D
is the forward voltage drop across the re-circulating
Q
is the voltage drop across the internal
Q
according to the FET current. F is
DS(ON)
of the FET is
8
Minimum Duty-Cycle
Besides a minimum off-time, there is also a minimum on-time
which will take effect when the output voltage is adjusted very
low and the input voltage is very high. Should the operation
require a shorter minimum on-time than the typical 100 ns,
individual switching pulses will be skipped.
where D is the duty-cycle.
Current Limit
When the power switch turns on, the slight capacitance load-
ing of the Schottky diode, D1, causes a leading-edge current
spike with an extended ringing period. This spike can cause
the current limit comparator to trip prematurely. A leading
edge blanking time (T
sampling the spike.
When the switch current reaches the current limit threshold,
the switch is immediately turned off and the internal switching
frequency is reduced. This extends the off time of the switch
to prevent a steady state high current condition. As the switch
current falls below the current limit threshold, the switch cur-
rent will attempt to turn on. If a load fault continues, the switch
will again exceed the threshold and turn off. This will result in
a low duty-cycle pulsing of the power switch to minimize the
overall fault condition power dissipation.
The switching frequency will reduce (fold back) if the overload
condition causes the output voltage to be 72.4% (typical) of
the adjusted output voltage.
The current limit will only protect the inductor from a runaway
condition if the LM22676 is operating in its safe operating
area. A runaway condition of the inductor is potentially catas-
trophic to the application. For every design, the safe operating
area needs to be calculated. Factors in determining the safe
operating area are the switching frequency, input voltage,
output voltage, minimum on-time and feedback voltage dur-
ing an over current condition.
As a first pass check, if the following equation holds true, a
given design is considered in a safe operating area and the
current limit will protect the circuit:
If the equation above does not hold true, the following sec-
ondary equation will need to hold true to be in safe operating
area:
If both equations do not hold true, a particular design will not
have an effective current limit function which might damage
the circuit during startup, over current conditions, or steady
state over current and short circuit condition. Oftentimes a
reduction of the maximum input voltage will bring a design into
the safe operating area.
Soft-Start
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
stresses and surges. The soft-start is fixed to 500 µs (typical)
start-up time and cannot be modified.
V
IN
x T
BLK
BLK
) of 110 ns (typical) is used to avoid
x F < V
OUT
x 0.724