SM72485/NOPB National Semiconductor, SM72485/NOPB Datasheet - Page 7

• Manufacturer Part Number: SM72485/NOPB
• Description: IC BUCK SWITCH 100V REG 8-LLP
• Manufacturer: National Semiconductor
• Series: SolarMagic™r
• Type: Step-Down (Buck)r

Specifications of SM72485/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 2.5V
• Current - Output: 150mA
• Frequency - Switching: 50kHz ~ 1.1MHz
• Voltage - Input: 6 V ~ 95 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The SM72485 regulates the output voltage based on ripple
voltage at the feedback input, requiring a minimum amount of
ESR for the output capacitor C2. A minimum of 25mV to 50mV
of ripple voltage at the feedback pin (FB) is required for the
SM72485. In cases where the capacitor ESR is too small,
additional series resistance may be required (R3 in the Block
Diagram).
Start-Up Regulator (V
The high voltage bias regulator is integrated within the
SM72485. The input pin (VIN) can be connected directly to
line voltages between 6V and 95V, with transient capability to
100V. Referring to the block diagram and the graph of V
V
inally 8.5V), the bypass switch (Q2) is on, and V
V
tance is approximately 100Ω, with inherent current limiting at
approximately 100 mA. When V
old Q2 is turned off, and V
regulator output current is limited at approximately 9.2 mA.
When the SM72485 is shutdown using the RT/SD pin, the
V
V
When VIN exceeds the bypass threshold, the time required
for Q2 to shut off is approximately 2 - 3 µs. The capacitor at
VCC (C3) must be a minimum of 0.47 µF to prevent the volt-
age at VCC from rising above its absolute maximum rating in
IN
IN
CC
IN
, when V
.
within 100 mV to 150 mV. The bypass switch on-resis-
bypass switch is shut off regardless of the voltage at
IN
is between 6V and the bypass threshold (nom-
CC
IN
is regulated at 7V. The V
is above the bypass thresh-
CC
)
FIGURE 1. Low Ripple Output Configuration
CC
tracks
CC
vs
CC
7
For applications where lower output voltage ripple is required
the output can be taken directly from a low ESR output ca-
pacitor, as shown in
the load regulation.
response to a step input applied at V
as close as possible to the VCC and RTN pins. In applications
with a relatively high input voltage, power dissipation in the
bias regulator is a concern. An auxiliary voltage of between
7.5V and 14V can be diode connected to the VCC pin to shut
off the V
pation. The current required into the VCC pin is shown in the
graph “I
connects VCC to VIN requiring that the auxiliary voltage be
less than V
The turn-on sequence is shown in
delay (t1) VCC ramps up at a rate determined by its current
limit and C3 while internal circuitry stabilizes. When V
reaches the upper threshold of its under-voltage lock-out (UV-
LO, typically 5.3V) the buckswitch is enabled. The inductor
current increases to the current limit threshold (I
ing t2 V
When V
tor current decreases (t3) to the nominal load current (I
CC
OUT
OUT
CC
Current vs. Applied V
IN
regulator, thereby reducing internal power dissi-
reaches the intended voltage the average induc-
increases as the output capacitor charges up.
.
Figure
1. However, R3 slightly degrades
30142313
CC
Voltage”. Internally a diode
Figure
IN
. C3 must be located
2. During the initial
LIM
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) and dur-
O
).
CC