Over-Voltage Comparator

The over-voltage comparator is provided to protect the out-

put from overvoltage conditions due to sudden input line

voltage changes or output loading changes. The over-

voltage comparator monitors the FB pin versus an internal

2.875V reference. If the voltage at FB rises above 2.875V

the comparator immediately terminates the buck switch on-

time pulse.

ON Time generator and Shutdown

The on-time of the LM25007 is set inversely proportional to

the input voltage by an external resistor connected between

RON/SD and VIN. The RON/SD terminal is a low impedance

input biased at approximately 1.5V. The current through the

resistor and into the RON/SD pin is approximately propor-

tional to Vin and used internally to control the on-timer. This

scheme of input voltage feed-forward hysteretic operation

achieves nearly constant operational frequency over varying

line and load conditions. The on-time equation for the

LM25007 is :

Ton=1.42x10

The RON/SD pin of the LM25007 also provides a shutdown

function which disables the regulator and significantly de-

creases quiescent power dissipation. By pulling the pin be-

low 0.7V activates the low power shutdown mode. The V

IN

quiescent current in the shutdown mode is approximately

100µA internal to the LM25007 plus the current in the R

ON

resistor. See Figure 3

Current Limit

The LM25007 contains an intelligent current limit off-timer

intended to reduce the foldback characteristic inherent with

fixed off-time over-current protection. If the current in the

Buck switch exceeds 725mA the present cycle on-time is

immediately terminated (cycle by cycle current limit). Follow-

ing the termination of the cycle a non-resetable current limit

off timer is initiated. The duration of the off time is a function

of the external resistor (RCL) and the FB pin voltage. When

the FB pin voltage equals zero, the current limit off-time is

internally preset to 17µs. This condition occurs in short cir-

cuit operation when a maximum amount of off-time is re-

quired.

In cases of overload (not complete short circuit) the current

limit off-time is reduced as a function of the output voltage

(measured at the FB pin). Reducing the off-time with smaller

overloads reduces the amount of foldback and also reduces

the initial start-up time. The current limit off time for a given

FB pin voltage and R

following equation:

Toff=10

Applications utilizing low resistance inductors and/or a low

voltage drop rectifier may require special evaluation at high

line, short circuit conditions. In this special case the preset

17µs (FB = 0V) off-time may be insufficient to balance the

inductor volt*time product. Additional inductor resistance,

output resistance or a larger voltage drop rectifier may be

necessary to balance the inductor cycle volt*time product

and limit the short circuit current.

N - Channel Buck Switch and

Driver

The LM25007 integrates an N-Channel Buck switch and

associated floating high voltage gate driver. The gate driver

circuit works in conjunction with an external bootstrap ca-

pacitor and an internal high voltage diode. The bootstrap

capacitor is charged by V

diode. A 0.01uF ceramic capacitor connected between the

BST pin and SW pin is recommended.

During each cycle when the Buck switch turns off, the SW

pin is approximately 0V. When the SW pin voltage is low, the

bootstrap capacitor is charged from Vcc through the internal

diode. The minimum off timer, set to 300ns, ensures that

there will be a minimum interval every cycle to recharge the

bootstrap capacitor.

20165507

FIGURE 3. Shutdown Implementation

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