LM5010 datasheet(10/19 Pages) NSC

10 / 19 page

Hysteretic Control Circuit

Overview (Continued)

Typically when the load current increases suddenly, the off-

times are temporarily at the minimum of 265 ns. Once regu-

lation is established, the off-time resumes its normal value.

The output voltage is set by two external resistors (R1, R2).

The regulated output voltage is calculated as follows:

OUT = 2.5V x (R1 + R2) / R2

(1)

Output voltage regulation is based on ripple voltage at the

feedback input, requiring a minimum amount of ESR for the

output capacitor C2. The LM5010 requires a minimum of 25

mV of ripple voltage at the FB pin. In cases where the

capacitor’s ESR is insufficient additional series resistance

may be required (R3 in Figure 1 ).

When in regulation, the LM5010 operates in continuous

conduction mode at heavy load currents and discontinuous

conduction mode at light load currents. In continuous con-

duction mode current always flows through the inductor,

never reaching zero during the off-time. In this mode the

operating frequency remains relatively constant with load

and line variations. The minimum load current for continuous

conduction mode is one-half the inductor’s ripple current

amplitude. The approximate operating frequency is calcu-

lated as follows:

(2)

The buck switch duty cycle is approximately equal to:

(3)

At low load current, the circuit operates in discontinuous

conduction mode, during which the inductor current ramps

up from zero to a peak during the on-time, then ramps back

to zero before the end of the off-time. The next on-time

period starts when the voltage at FB falls below the refer-

ence - until then the inductor current remains zero, and the

load current is supplied by the output capacitor (C2). In this

mode the operating frequency is lower than in continuous

conduction mode, and varies with load current. Conversion

efficiency is maintained at light loads since the switching

losses reduce with the reduction in load and frequency. The

approximate discontinuous operating frequency can be cal-

culated as follows:

(4)

where R

L = the load resistance.

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 9. However, R3 slightly

degrades the load regulation.

Start-up Regulator (V

CC)

The startup regulator is integral to the LM5010. The input pin

IN) can be connected directly to line voltages up to 75V.

The V

CC output is regulated at 7.0V, ±6%, and is current

limited to 10 mA. Upon power up the regulator sources

current into the external capacitor at V

CC (C3). With a 0.1 µF

capacitor at V

CC, approximately 58 µs are required for the

CC voltage to reach the under-voltage lockout threshold

(UVLO) of 5.8V (t1 in Figure 8), at which time the buck switch

is enabled, and the softstart pin is released to allow the

softstart capacitor (C6) to charge up. V

OUT then increases to

its regulated value as the softstart voltage increases (t2 in

Figure 8).

The minimum input operating voltage is determined by the

regulator’s dropout voltage, the V

CC UVLO falling threshold

()5.65V), and the frequency. When V

CC falls below the

falling threshold the V

CC UVLO activates to shut off the buck

switch and ground the softstart pin. If V

CC is externally

loaded, the minimum input voltage increases since the out-

put impedance at V

CC is

)140

Ω at low V

IN. See Figures 2

and 3. In applications involving a high value for V

IN where

power dissipation in the startup regulator is a concern, an

auxiliary voltage can be diode connected to the V

CC pin

(Figure 10). Setting the auxiliary voltage to between 7.5V

and 14V shuts off the internal regulator, reducing internal

power dissipation. The current required into the V

CC pin is

shown in Figure 4. Internally a diode connects V

CC to VIN.

20119915

FIGURE 9. Low Ripple Output Configuration

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