lm22671mrx-adj National Semiconductor Corporation, lm22671mrx-adj Datasheet - Page 10

• Manufacturer Part Number: lm22671mrx-adj
• Description: 500 Ma Simple Switcher , Step-down Voltage Regulator With Features
• Manufacturer: National Semiconductor Corporation

www.national.com
Thermal Protection
Internal Thermal Shutdown circuitry protects the LM22671 in
the event the maximum junction temperature is exceeded.
When activated, typically at 150°C, the regulator is forced into
a low power reset state. There is a typical hysteresis of 15
degrees.
Internal Compensation
The LM22671 has internal compensation designed for a sta-
ble loop with a wide range of external power stage compo-
nents.
Insuring stability of a design with a specific power stage (in-
ductor and output capacitor) can be tricky. The LM22671
stability can be verified over varying loads and input and out-
put voltages using WEBENCH® Designer online circuit sim-
ulation tool at www.national.com. A quick start spreadsheet
can also be downloaded from the online product folder.
The internal compensation of the -ADJ option of the LM22671
is optimized for output voltages below 5V. If an output voltage
of 5V or higher is needed, the -5.0 option with an additional
external resistor divider may also be used.
The typical location of the internal compensation poles and
zeros as well as the DC gain is given in Table 1. The LM22671
has internal type III compensation allowing for the use of most
output capacitors including ceramics.
This information can be used to calculate the transfer function
from the FB pin to the internal compensation node (input to
the PWM comparator in the block diagram).
For the power stage transfer function the standard voltage
mode formulas for the double pole and the ESR zero apply:
The peak ramp level of the oscillator signal feeding into the
PWM comparator is V
this modulator stage of the IC. The -5.0 fixed output voltage
option has twice the gain of the compensation transfer func-
tion compared to the -ADJ option which is 43.5dB instead of
37.5dB.
Generally, calculation as well as simulation can only aid in
selecting good power stage components. A good design prac-
tice is to test for stability with load transient tests or loop
measurement tests. Application note AN-1889 shows how to
easily perform a loop transfer function measurement with only
an oscilloscope and a function generator.
Corners
DC gain
Zero 1
Zero 2
Pole 1
Pole 2
Pole 3
IN
/10 which equals a gain of 20dB of
TABLE 1.
Frequency
150 kHz
250 kHz
1.5 kHz
37.5 dB
100 Hz
15 kHz
10
Application Information
EXTERNAL COMPONENTS
The following design procedures can be used to design a non-
synchronous buck converter with the LM22671.
Inductor
The inductor value is determined based on the load current,
ripple current, and the minimum and maximum input voltage.
To keep the application in continuous current conduction
mode (CCM), the maximum ripple current, I
less than twice the minimum load current.
The general rule of keeping the inductor current peak-to-peak
ripple around 30% of the nominal output current is a good
compromise between excessive output voltage ripple and ex-
cessive component size and cost. When selecting the induc-
tor ripple current ensure that the peak current is below the
minimum current limit as given in the Electrical Characteris-
tics section. Using this value of ripple current, the value of
inductor, L, is calculated using the following formula:
where F is the switching frequency which is 500 kHz without
an external frequency set resistor or external sync signal ap-
plied to the RT/SYNC pin. If the switching frequency is set
higher than 500 kHz, the inductance value may not be re-
duced accordingly due to stability requirements. The internal
compensation is optimized for circuits with a 500 kHz switch-
ing frequency. See the internal compensation section for
more details. This procedure provides a guide to select the
value of the inductor L. The nearest standard value will then
be used in the circuit.
Increasing the inductance will generally slow down the tran-
sient response but reduce the output voltage ripple amplitude.
Reducing the inductance will generally improve the transient
response but increase the output voltage ripple.
The inductor must be rated for the peak current, I
vent saturation. During normal loading conditions, the peak
current occurs at maximum load current plus maximum ripple.
Under an overload condition as well as during load transients,
the peak current is limited to 700 mA typical (900 mA maxi-
mum). This requires that the inductor be selected such that it
can run at the maximum current limit and not only the steady
state current.
Depending on inductor manufacturer, the saturation rating is
defined as the current necessary for the inductance to reduce
by 30% at 20°C. In typical designs the inductor will run at
higher temperatures. If the inductor is not rated for enough
current, it might saturate and due to the propagation delay of
the current limit circuitry, the power supply may get damaged.
Input Capacitor
Good quality input capacitors are necessary to limit the ripple
voltage at the VIN pin while supplying most of the switch cur-
rent during on-time. When the switch turns on, the current into
the VIN pin steps to the peak value, then drops to zero at turn-
off. The average current into VIN during switch on-time is the
load current. The input capacitance should be selected for
RMS current, I
proximation for the required ripple current rating necessary is
I
RMS
> I
OUT
/ 2.
RMS
, and minimum ripple voltage. A good ap-
RIPPLE
, should be
PK+
, to pre-