LM3477MM National Semiconductor, LM3477MM Datasheet - Page 15

• Manufacturer Part Number: LM3477MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3477MM

• Input Voltage Primary Min: 2.95V
• Mounting Type: Surface Mount
• Topology: Buck (Step Down)
• Control Mode: Current
• Duty Cycle Max: 93%
• Input Voltage Primary Max: 35V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Design Section
Example: V
The hysteretic threshold is derived in a similar manner, the
only difference being that V
(V
The hysteretic threshold is predetermined by the selection of
R
Continuing with the example above,
If the peak switch current decreases below this threshold,
the LM3477/A will operate in hysteretic mode (see OVER
VOLTAGE PROTECTION section). In some designs, it will
be desired to use R
used (see DEFAULT/ADJUSTABLE SLOPE COMPENSA-
TION section and Inductor section). Using R
current limit and the hysteretic threshold. See Figure 8. R
effectively adds an additional slope to the existing slope of
the V
When R
FIGURE 8. Current Limit and Hysteretic Threshold vs
SN
HYS
above. The hysteretic threshold is:
C
). Notice that V
waveform.
SL
is used, the following equations apply:
IN(MIN)
= 4.5V, V
Duty Cycle with R
SL
HYS
so that lower valued inductors can be
does not vary with the duty cycle.
(Continued)
OUT
SEN(peak)
= 2.5V, I
SL
is compared V
OUT(MAX)
SL
will lower the
200033K4
= 3A
C(min)
SL
15
where MIN(V
the two values in the parenthesis and V
0.011V for the LM3477 and LM3477A, respectively. R
be used creatively to intentionally lower the hysteretic
threshold, allowing for better performance at lower loads.
However, when R
requirement (see START-UP/SOFT-START section).
Power Inductor Section
The LM3477/A operates at a high switching frequency of
500kHz, which allows the use of small inductors. This is
made apparent in the following set of equations used to
calculate the output voltage ripple.
As the switching frequency fs increases, the inductance
required for a given output voltage ripple decreases. The
equations above for ∆V
choosing the inductance. The maximum voltage ripple in
steady-state, PWM operation can be controlled by limiting
∆i
one can simply choose ∆i
output current. Clearly, the size of the output capacitor ESR,
R
the inductance. When the ESR is relatively low (less than
100mΩ), such as in ceramic, OSCON, and some low ESR
tantalum capacitors, it is convenient to choose the induc-
tance based on setting ∆i
high, then it may be necessary to restrict ∆i
so that the output voltage ripple is not too high. Generally
speaking, the former suggestion of setting ∆i
I
The inductance also affects the stability of the converter. The
slopes S
while the compensation ramp, S
fore if the inductance is too small, the converter may expe-
rience sub-harmonic oscillations. The LM3477/A provides
sufficient internal slope compensation to allow for induc-
tances chosen according to the ∆i
most cases. Still, one should check to make sure the induc-
tance is not too low before continuing the design process. If
it is found that the selected inductance is too low, a patented
scheme to increase the compensation ramp, S
in the LM3477/A (see DEFAULT/ADJUSTABLE SLOPE
COMPENSATION section). In the calculations that follow, if it
is found that the chosen inductance is too small, R
used to increase Se so that the inductance can be used.
In a current mode control architecture, there is an inherent
resonance
DEFAULT/ADJUSTABLE SLOPE COMPENSATION sec-
OUT(MAX)
ESR
L
which in turn is set by the inductance value. Alternatively,
, will have an affect on which criteria is used to choose
n
is recommended.
and S
∆V
HYS
at
OUT(Pk-Pk)
f
− 50x10
in Figure 4 are functions of the inductance,
SL
half
is used, there may be a minimum load
the
) ∆i
L
-6
L
OUT
to 30% of Iout(max). If the ESR is
as a percentage of the maximum
x R
L(Pk-Pk)
SL
switching
and ∆i
e
, is fixed by default. There-
x D
L
= 0.3 x I
MAX
x R
L
provide criteria for
, 0) is the smaller of
ESR
HYS
L
frequency
OUT
to a lower value
(V)
is 0.032V and
e
L
, is provided
guideline in
www.national.com
to 30% of
SL
can be
SL
(see
can