LM3477MM National Semiconductor, LM3477MM Datasheet - Page 12

• 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
calculations for minimum inductance and necessary slope
resistance R
ing.
START-UP/SOFT-START
The LM3477/A incorporates an internal soft-start during
start-up. The soft-start forces the inductor current to rise
slowly and smoothly as it increases towards the steady-state
current. This technique is used to reduce the input inrush
current during soft-start. The soft-start functionality is effec-
tive for approximately the first 5ms of start-up.
Note: The LM3477/A will not start-up if the output voltage is biased by more
Note: If the slope resistor R
SHORT CIRCUIT PROTECTION
When the voltage across the sense resistor (measured as
the V
current limit gets activated. In the short-circuit protection
mode, the external MOSFET is turned off. When the short is
removed, the external MOSFET is turned on after five
cycles. The short circuit protection voltage V
the ELECTRICAL CHARACTERISTICS section. V
lower in the LM3477A than in the LM3477.
SHUTDOWN
The compensation pin (Pin 2) of LM3477/A also functions as
a shutdown pin. If a low signal (refer to the ELECTRICAL
CHARACTERISTICS for definition of low signal) appears on
the COMP/SD pin, the LM3477/A stops switching and goes
into a low supply current mode. The total supply current of
the IC reduces to less than 10µA under these conditions.
Figure 5 shows different implementations of the shutdown
function.
than 200mV above ground.
lowered. Therefore, the LM3477/A may require up to 100mA of pre-
load to successfully start up.
IN
− I
SEN
SL
differential voltage) exceeds V
are carried out based on this resonant peak-
SL
is used, the hysteretic threshold will be
(Continued)
SC
SC
is specified in
, short-circuit
SC
is
12
Design Section
GENERAL
Power supply design involves making tradeoffs. To achieve
performance specifications, limitations will be set on compo-
nent selection. The LM3477/A provides many degrees of
flexibility in choosing external components to accommodate
various performance/component selection optimizations. For
example, the internal slope compensation can be externally
increased to allow smaller inductances to be used. The
design procedures that follow provide instruction on how to
select the external components in a typical LM3477/A buck
circuit in continuous conduction mode, as well as aid in the
optimization of performance and/or component selection.
See Figure 6 for component reference and typical circuit.
The LM3477/A may also be designed to operate in discon-
tinuous conduction mode.
FIGURE 5. Implementing Shutdown in LM3477
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