Regulation Comparator

The feedback voltage at the FB pin is compared to the

on-time period is initiated when the voltage at FB falls below

2.5V. The buck switch conducts for the on-time programmed

by R

on-time period the buck switch remains off until the FB

voltage falls below 2.5V. Input 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 2.9V

reference. If the voltage at FB rises above 2.9V the on-time

is immediately terminated. This condition can occur if the

input voltage, or the output load, changes suddenly. The

buck switch remains off until the voltage at FB falls below

2.5V.

ON-Time Control

The on-time of the internal buck switch is determined by the

R

follows:

(5)

The R

time by re-arranging Equation 5 to the following:

(6)

To set a specific continuous conduction mode switching

frequency (Fs), the R

following:

(7)

In high frequency applications the minimum value for t

limited by the maximum duty cycle required for regulation

The fixed off-time limits the maximum duty cycle achievable

with a low voltage at VIN. The minimum allowed on-time to

regulate the desired V

from the following:

(8)

Shutdown

The LM5010A can be remotely shut down by forcing the

RON/SD pin below 0.7V with a switch or open drain device.

See Figure 3. In the shutdown mode the SS pin is internally

grounded, the on-time one-shot is disabled, the input current

at VIN is reduced, and the V

The V

Releasing the RON/SD pin allows normal operation to re-

sume. The nominal voltage at RON/SD is shown in the

Typical Performance Characteristics. When switching the

RON/SD pin, the transition time should be faster than one to

two cycles of the regulator’s nominal switching frequency.

Current Limit

Current limit detection occurs during the off-time by monitor-

ing the recirculating current through the internal current

sense resistor (R

SGND during the off-time exceeds the threshold level the

current limit comparator delays the start of the next on-time

period. The next on-time starts when the current into SGND

is below the threshold and the voltage at FB is below 2.5V.

Figure 4 illustrates the inductor current waveform during

normal operation and during current limit. The output current

I

The Low Load Current waveform illustrates continuous con-

duction mode operation with peak and valley inductor cur-

rents below the current limit threshold. When the load cur-

rent is increased (High Load Current), the ripple waveform

maintains the same amplitude and frequency since the cur-

rent falls below the current limit threshold at the valley of the

ripple waveform. Note the average current in the High Load

Current portion of Figure 4 is above the current limit thresh-

old. Since the current reduces below the threshold in the

normal off-time each cycle, the start of each on-time is not

delayed, and the circuit’s output voltage is regulated at the

correct value. When the load current is further increased

such that the lower peak would be above the threshold, the

off-time is lengthened to allow the current to decrease to the

threshold before the next on-time begins (Current Limited

portion of Figure 4). Both V

are reduced as the circuit operates in a constant current

20153818

FIGURE 3. Shutdown Implementation

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