LM3203 datasheet(12/15 Pages) NSC

12 / 15 page

Shutdown Mode

Setting the EN digital pin low (<0.4V) places the LM3203 in

a 0.1µA (typ.) Shutdown mode. During shutdown, the PFET

switch, NFET synchronous rectifier, reference voltage

source, control and bias circuitry of the LM3203 are turned

off. Setting EN high (>1.2V) enables normal operation.

EN should be set low to turn off the LM3203 during power-up

and under voltage conditions when the power supply is less

than the 2.7V minimum operating voltage. The LM3203 is

designed for compact portable applications, such as mobile

phones. In such applications, the system controller deter-

mines power supply sequencing and requirements for small

package size outweigh the benefit of including UVLO (Under

Voltage Lock-Out) circuitry.

Dynamically Adjustable Output

Voltage

The LM3203 features dynamically adjustable output voltage.

The output can be set by changing the voltage on the analog

CON pin. This feature is useful in PA applications where

peak power is needed only when the handset is far away

from the base station or when data is being transmitted. In

other instances, the transmitting power can be reduced.

Hence the supply voltage to the PA can be reduced, promot-

ing longer battery life. See Setting the Output Voltage in the

Application Information section for further details.

Over Voltage Protection

The LM3203 has an over voltage comparator that prevents

the output voltage from rising too high, when the device is

left in PWM mode under light-load conditions, during output

voltage steps, or during startup. When the FB pin voltage

rises by 100mV over the V

CON voltage, the OVP comparator

inhibits PWM operation to skip pulses until the feedback

voltage returns to the V

CON voltage. During the over voltage

protection mode, both the PWM PFET and the NFET syn-

chronous rectifier are off. When the part comes out of the

over-voltage protection mode, the NFET synchronous recti-

fier remains off for approximately 3.5µs to avoid inductor

current going negative.

Internal Synchronous Rectification

While in PWM mode, the LM3203 uses an internal NFET as

a synchronous rectifier to reduce rectifier forward voltage

drop and associated power loss. Synchronous rectification

provides a significant improvement in efficiency whenever

the output voltage is relatively low compared to the voltage

drop across an ordinary rectifier diode.

With medium and heavy loads, the internal NFET synchro-

nous rectifier is turned on during the inductor current down

slope in the second part of each cycle. The synchronous

rectifier is turned off prior to the next cycle. There is no zero

cross detect, which means that the NFET can conduct cur-

rent in both directions and inductor current is always con-

tinuous. The advantage of this method is that the part re-

mains in PWM mode at light loads or no load conditions. The

NFET has a current limit. The NFET is designed to conduct

through its intrinsic body diode during transient intervals

before it turns on, eliminating the need for an external diode.

Current Limiting

A current limit feature allows the LM3203 to protect itself and

external components during overload conditions. In PWM

mode, a 940mA (max.) cycle-by-cycle current limit is nor-

mally used. If an excessive load pulls the output voltage

down to below approximately 0.375V, indicating a possible

short to ground, then the device switches to a timed current

limit mode. In timed current limit mode, the internal PFET

switch is turned off after the current comparator trips, and the

beginning of the next cycle is inhibited for 3.5µs to force the

instantaneous inductor current to ramp down to a safe value.

After the 3.5µs interval, the internal PFET is turned on again.

This cycle is repeated until the load is reduced and the

output voltage exceeds approximately 0.375V. Therefore,

the device may not startup if an excessive load is connected

to the output when the device is enabled. The synchronous

rectifier is off in the timed current limit mode. Timed current

limit prevents the loss of current control seen in some prod-

ucts when the output voltage is pulled low in serious over-

load conditions. This timed current limit function is disabled if

the BYPOUT pin is connected to V

DD.

A current limit is also provided for the NFET. This is approxi-

mately −500mA. Both the NFET and the PFET are turned off

in negative current limit until the PFET is turned on again at

the beginning of the next cycle. The negative current limit

inhibits buildup of excessive negative inductor current.

In the Bypass mode, the bypass current limit is 1000mA

(typ.). The output voltage drops when the bypass current

limit kicks in.

Thermal Overload Protection

The LM3203 has a thermal overload protection function to

protect the device from short-term misuse and overload

conditions. When the junction temperature exceeds around

150˚C, the device inhibits operation. Both the PFET and the

NFET are turned off in PWM mode, and the Bypass PFET is

turned off in Bypass mode. When the temperature drops

below 130˚C, normal operation resumes. Prolonged opera-

tion in thermal overload conditions may damage the device.

Application Information

SETTING THE OUTPUT VOLTAGE

The LM3203 features a pin-controlled variable feedback volt-

age. It can be programmed for an output voltage from 0.8V

to 3.6V by setting the voltage on the V

CON pin, as in the

formula below ( Refer Figure 1). In this scheme the output

voltage is directly proportional to V

CON.

OUT =((R1+R2)/R2)xVCON

When V

CON is between 0.267V and 1.20V, the output volt-

age will follow the above equation. For example, the V

OUT is

proportionally 3 times of V

CON when external resistor divid-

ers are used as in Figure 1 .

If V

OUT is over 3.6V, sub-harmonic oscillation may occur

because of insufficient slope compensation.

If the target voltage is less than 0.8V, the output voltage may

not be regulated due to the required on-time being less than

the minimum on-time (50ns). The output voltage can go

lower than 0.8V providing a limited V

IN range is used. Refer

to datasheet curve (Low V

CON Voltage vs Output Voltage) for

details. This curve is for a typical part and there could be part

to part variation for output voltages less than 0.8V over the

limited V

IN range. In addition, if the VCON voltage is less than

approx. 0.15V, the LM3203 output is off, but the internal bias

circuits are still active.

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