Hysteretic Control Circuit

(1)

The output voltage (V

resistors as shown in Figure 1. The regulation point is cal-

culated as follows:

V

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.

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:

T

(2)

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

20165805

FIGURE 2. Low Ripple Output Configuration

20165806

FIGURE 3. Self Biased Configuration

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