LM5009MM National Semiconductor, LM5009MM Datasheet - Page 8

• Manufacturer Part Number: LM5009MM
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5009MM

• Rohs Compliant: NO

www.national.com
Hysteretic Control Circuit
Overview
The output voltage (V
resistors as shown in Figure 1. The regulation point is cal-
culated as follows:
All hysteretic regulators regulate 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 of ripple voltage at the feedback pin (FB) is required
for the LM5009. In cases where the capacitor ESR is too
small, additional series resistance may be required (R3 in
Figure 1).
For applications where lower output voltage ripple is re-
quired the output can be taken directly from a low ESR
output capacitor, as shown in Figure 2. However, R3 slightly
degrades the load regulation.
High Voltage Start-Up Regulator
The LM5009 contains an internal high voltage startup regu-
lator. The input pin (VIN) can be connected directly to line
voltages up to 95 Volts, with transient capability to 100 volts.
The regulator is internally current limited at 9.5mA. Upon
power up, the regulator sources current into the external
capacitor at VCC (C3). When the voltage on the VCC pin
reaches the under-voltage lockout threshold of 6.3V, the
buck switch is enabled.
In applications involving a high value for V
dissipation in the V
voltage can be diode connected to the VCC pin. Setting the
voltage between 8V and 14V shuts off the internal regulator,
reducing internal power dissipation. See Figure 3. The cur-
rent required into the VCC pin is shown in the Typical Per-
formance Characteristics.
FIGURE 2. Low Ripple Output Configuration
V
OUT
(Continued)
CC
= 2.5 x (R1 + R2) / R2
OUT
regulator is a concern, an auxiliary
) is programmed by two external
IN
, where power
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Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V
reference. In normal operation (the output voltage is regu-
lated), an on-time period is initiated when the voltage at FB
falls below 2.5V. The buck switch will stay on for the pro-
grammed on-time, causing the FB voltage to rise above
2.5V. After the on-time period, the buck switch will stay off
until the FB voltage again falls below 2.5V. During start-up,
the FB voltage will be below 2.5V at the end of each on-time,
resulting in the minimum off-time. Bias current at the FB pin
is less than 5 nA over temperature.
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal
2.875V reference. If the voltage at FB rises above 2.875V
the on-time pulse is immediately terminated. This condition
can occur if the input voltage, or the output load, change
suddenly. The buck switch will not turn on again until the
voltage at FB falls below 2.5V.
ON-Time Generator and Shutdown
The on-time for the LM5009 is determined by the R
tor, and is inversely proportional to the input voltage (Vin),
resulting in a nearly constant frequency as Vin is varied over
its range. The on-time equation is:
R
V
This requirement limits the maximum frequency for each
application, depending on V
The LM5009 can be remotely disabled by taking the
RON/SD pin to ground. See Figure 4. The voltage at the
RON/SD pin is between 1.7V and 5V, depending on Vin and
the value of the R
IN
ON
) greater than 250 ns, for proper current limit operation.
should be selected for a minimum on-time (at maximum
FIGURE 3. Self Biased Configuration
T
ON
ON
= 1.25 x 10
resistor.
IN
-10
and V
x R
OUT
ON
.
/ V
IN
ON
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