from a pulse or function generator can develop under/

overshoot as high as 10V at the end of an improperly termi-

nated cable.

OVERVOLTAGE PROTECTION

The LM2619MTC has an over-voltage comparator that pre-

vents the output voltage from rising too high when the device

is left in PWM mode under low-load conditions. When the

output voltage rises by about 100mV (Figure 3) over its

regulation threshold, the OVP comparator inhibits PWM op-

eration to skip pulses until the output voltage returns to the

regulation threshold. When resistor dividers are used the

OVP threshold at the output will be the value of the threshold

at the feedback pin times the resistor divider ratio. In over

voltage protection, output voltage and ripple will increase.

SHUTDOWN MODE

LM2619MTC in a 0.02µA (typ) shutdown mode. During shut-

down, the PFET switch, NFET synchronous rectifier, refer-

ence, control and bias circuitry of the LM2619MTC are

turned off. Setting EN high enables normal operation. While

turning on, soft start is activated.

EN should be set low to turn off the LM2619MTC during

system power-up and undervoltage conditions when the

supply is less than the 2.8V minimum operating voltage. The

LM2619MTC is designed for compact portable applications,

such as mobile phones. In such applications, the system

controller determines power supply sequencing. Although

the LM2619MTC is typically well behaved at low input volt-

ages, this is not guaranteed.

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM2619MTC uses an internal

NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rec-

tification provides a significant improvement in efficiency

whenever the output voltage is relatively low compared to

the voltage drop across an ordinary rectifier diode.

The internal NFET synchronous rectifier is turned on during

the inductor current down slope during the second part of

each cycle. The synchronous rectifier is turned off prior to the

next cycle, or when the inductor current ramps to zero at light

loads. The NFET is designed to conduct through its intrinsic

body diode during transient intervals before it turns on, elimi-

nating the need for an external diode.

CURRENT LIMITING

A current limit feature allows the LM2619MTC to protect

itself and external components during overload conditions. In

PWM mode cycle-by-cycle current limit is normally used. If

an excessive load pulls the voltage at the feedback pin down

to approximately 0.7V, then the device switches to a timed

current limit mode. In timed current limit mode the internal

P-FET switch is turned off after the current comparator trips

and the beginning of the next cycle is inhibited for 2.5µs to

force the instantaneous inductor current to ramp down to a

safe value. Timed current limit mode prevents the loss of

current control seen in some products when the voltage at

the feedback pin is pulled low in serious overload conditions.

DROPOUT CONSIDERATIONS

The LM2619MTC can be used to provide fixed output volt-

ages by using external feedback resistors. The output volt-

age can be set from 1.5V to 3.6V. The internal reference

voltage for the error amplifier is 1.5V. In cases where the

output voltage is set to 2.5V or higher, the part will go into

dropout or 100% duty cycle when the input voltage gets

close to the set output voltage. Near dropout the on time of

the P-FET may exceed one PWM clock cycle and cause

higher ripple on the output for load currents greater than

300mA. This increased ripple will exist for a narrow range of

input voltages close to the 100% duty cycle and once the

input voltage goes down further the P-FET will be fully on.

See Setting the Output Voltage in the Application Information

section for further details.

In dropout conditions the output voltage is V

R

and R

Load Transient Response

(Circuit in Figure 3)

20065127

FIGURE 8.

Line Transient Response

(Circuit in Figure 3)

20065128

FIGURE 9.

www.national.com

12

PrintDate=2003/08/20 PrintTime=18:54:05 801627bc ds200651_p Rev. No. 1.25

cmserv

Proof

Seq=12