lm5006mmx National Semiconductor Corporation, lm5006mmx Datasheet - Page 11

• Manufacturer Part Number: lm5006mmx
• Description: 80v, 600 Ma Constant On-time Buck Switching Regulator
• Manufacturer: National Semiconductor Corporation

is maintained, since the switching losses reduce with the re-
duction in load and frequency. The discontinuous operating
frequency can be calculated as follows:
where R
In continuous conduction mode, current flows continuously
through the inductor and never ramps down to zero. In this
mode the operating frequency is greater than the discontinu-
ous mode frequency and remains relatively constant with load
and line variations. The approximate continuous mode oper-
ating frequency can be calculated as follows:
The buck switch duty cycle is approximately equal to:
Start-Up Regulator (V
The high voltage bias regulator is integrated within the
LM5006. The input pin (VIN) can be connected directly to line
voltages between 6V and 75V, with transient capability to
80V. The V
output current is limited at approximately 30 mA.
C3 must be located 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 10V can be diode connected to
the VCC pin to shut off the V
internal power dissipation. The current required into the VCC
pin depends on the voltage applied to VCC, the switching fre-
L
= the load resistance
CC
output is regulated at 7.5V. The V
CC
regulator, thereby reducing
CC
)
FIGURE 1. Low Ripple Output Configuration
CC
regulator
(1)
(2)
11
If a synchronous rectifier (Q1 in the Typical Application
schematic on the front page) is used instead of a flyback
diode, the LM5006 operates in continuous conduction mode
for all values of load current. The switching frequency remains
relatively constant with load and line variations, and can be
calculated using Equation 1.
The output voltage (V
sistors as shown in the Block Diagram. The regulation point
can be calculated as follows:
The LM5006 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
LM5006. In cases where the capacitor ESR is too small, ad-
ditional series resistance may be required (R3 in the Block
Diagram).
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.
quency, and whether a flyback diode (D1) or a synchronous
rectifier (Q1) is used. See the graph “I
V
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, the
buckswitch is enabled. The inductor current increases to the
current limit threshold (I
the output capacitor charges up. When V
tended voltage the average inductor current decreases (t3) to
the nominal load current (I
CC
.” Internally a diode connects VCC to VIN requiring that
V
OUT
= 2.5 x (R
Figure
OUT
LIM
) is programmed by two external re-
O
) and during t2 V
30113113
1. However, R3 slightly degrades
).
FB1
+ R
Figure
IN
FB2
CC
.
vs. Externally Applied
) / R
OUT
2. During the initial
OUT
FB1
reaches the in-
increases as
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CC