LM3477MM National Semiconductor, LM3477MM Datasheet - Page 10

• 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

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Functional Description
The important differences between the LM3477 and the
LM3477A are summarized in Figure 1. The voltages in Fig-
ure 1 can be referred to the switch current by dividing
through by R
old voltage V
larger load range than the LM3477. Typically, V
for the LM3477, while V
difference in area between the shaded regions give a graphi-
cal representation of this. The lightly shaded region is the
extra PWM operating area gained by using the LM3477A.
Thus the benefits of operating in PWM mode such as a well
regulated output voltage with low noise ripple are extended
to a larger load range when the LM3477A is used. While less
significant, the other noteworthy difference between the two
parts is in the short circuit current limit V
V
the SHORT CIRCUIT CURRENT LIMIT section). V
lower in the LM3477A than in the LM3477 (see the ELEC-
TRICAL CHARACTERISTICS section for limits).
OVER VOLTAGE PROTECTION
The LM3477/A has over voltage protection (OVP) for the
output voltage. OVP is sensed at and is in respect to the
feedback pin (pin 3). If at anytime the voltage at the feedback
pin rises to V
CHARACTERISTICS section for limits on V
OVP will cause the drive pin to go low, forcing the power
MOSFET off. With the MOSFET off, the output voltage will
drop. The LM3477/A will begin switching again when the
feedback voltage reaches V
ELECTRICAL CHARACTERISTICS for limits on V
OVP can be triggered by any event that causes the output
voltage to rise out of regulation. There are several common
circumstances in which this can happen, and it is beneficial
for a designer to be aware of these for debugging purposes,
since the mode of operation changes from the normal Pulse
Width Modulation (PWM) mode to the hysteretic mode. In
the hysteretic mode the output voltage is regulated between
a high and low value that results in a higher ripple magnitude
and lower ripple frequency than in the PWM mode, see
Figure 2.
SC
Output Voltage. See different Ripple Components in
FIGURE 2. The Feedback Voltage is related to the
is a ceiling limit for the peak sense voltage V
SN
FB
HYS
PWM and Hysteretic Modes
. The LM3477A has a lower hysteretic thresh-
+ V
, and thus will operate in PWM mode for a
OVP
, OVP is triggered. See ELECTRICAL
HYS
= 11mV for the LM3477A. The
FB
+ (V
OVP
SC
(Continued)
- V
.
FB
OVP(HYS)
200033C1
and V
HYS
SNpk
OVP(HYS)
= 32mV
OVP
). See
SC
(see
.
is
.
10
If the load current becomes too low, the LM3477/A will
increase the duty cycle, causing the voltage to rise and
trigger the OVP. The reasons for this involve the way the
LM3477/A regulates the output voltage, using a control
waveform at the pulse width modulator. This control wave-
form has upper and lower bounds.
Another way OVP can be tripped is if the input voltage rises
higher than the LM3477/A is able to regulate in pulse width
modulation (PWM) mode. The output voltage is related to
the input voltage by the duty cycle as: V
LM3477/A has a minimum duty cycle of 16.5% (typical), due
to the blank-out timing, TMIN. If the input voltage increases
such that the duty cycle wants to be less than D
cycle will hold at D
with the input voltage until it trips OVP.
It is useful to plot the operational boundaries in order to
illustrate the point at which the device switches into hyster-
etic mode. In Figure 1, the limits shown are with respect to
the peak voltage across the sense resistor R
they can be referred to the peak inductor current by dividing
through by R
normal circumstances V
region, and the LM3477/A will operate in the PWM mode. If
operating conditions are chosen such that V
normally fall in the shaded regions, then the mode of opera-
tion is changed so that V
and the part will operate in the hysteretic mode. What actu-
ally happens is that the LM3477/A will not allow V
outside of the shaded regions, so the duty cycle is adjusted.
The output voltage transient response overshoot can also
trigger OVP. As discussed in the OUTPUT CAPACITOR
section, if the capacitance is too low or ESR too high, the
output voltage overshoot will rise high enough to trigger
OVP. However, as long as there is room for the duty cycle to
adjust (the converter is not near D
LM3477/A will return to PWM mode after a few cycles of
hysteretic mode operation.
There is one last way that OVP can be triggered. If the
unregulated input voltage crosses 7.2V, the output voltage
will react as shown in Figure 3. The internal bias of the
LM3477/A switches supplies at 7.2V. When this happens, a
sudden small change in bias voltage is seen by all the
internal blocks of the LM3477/A. The control voltage, VC,
shifts because of the bias change, the PWM comparator
tries to keep regulation. To the PWM comparator, the sce-
nario is identical to step change in the load current, so the
response at the output voltage is the same as would be
observed in a step load change. Hence, the output voltage
overshoot here can also trigger OVP. The LM3477/A will
regulate in hysteretic mode for several cycles, or may not
recover and simply stay in hysteretic mode until the load
current drops. Note that the output voltage is still regulated in
hysteric mode. Predicting whether or not the LM3477/A will
come out of hysteretic mode in this scenario is a difficult
task, however it is largely a function of the output current and
the output capacitance. Triggering hysteretic mode in this
way is only possible at higher load currents. The method to
avoid this is to increase the output capacitance.
SN
. V
SNpk
MIN
and the output voltage will increase
SNpk
is bound to the shaded regions. In
SNpk
is required to be in the shaded
will be in the shaded region,
MIN
OUT
or D
SNpk
= V
MIN
SN
SNpk
MAX
would not
IN
, (V
, the duty
*D. The
), the
SNpk
to be
);