Data Sheet

LT83203/LT83205

analog.com

Rev. B

32 of 44

When the enable pin drops below 0.7V, the part stops switching, but internal circuitry continues drawing current as

the INTVCC regulator is still awake. Full shutdown is guaranteed when the enable pin drops below 200mV. In full

shutdown, the INTVCC regulator is disabled, and the part draws less than 70μA.

Adding a resistor divider from VIN to EN/UVLO programs the LT83203/LT83205 to regulate the output only when VIN is

above the required voltage (see the Block Diagram). This threshold, VIN(EN), is typically used when the input supply is

either current limited or has a relatively high source resistance. A switching regulator draws constant power from the

source, so the source current increases as the source voltage drops. This looks like a negative resistance load to the

source and causes the source to current limit or latch low under low source voltage conditions. The VIN(EN) threshold

prevents the regulator from operating at source voltages where the problems might occur. This threshold can be

adjusted by setting the values of REN1 and REN2 such that they satisfy Equation 10.

VIN(EN) = (

REN1

REN2

+ 1) × 0.75V

(10)

where the LT83203/LT83205 remains off until VIN is above VIN(EN). Due to the comparator’s hysteresis, switching does

not stop until the input falls slightly below VIN(EN).

INTV

CC Regulator

An internal low dropout (LDO) regulator produces the 3.4V supply from VIN that powers the drivers and the internal

bias circuitry. The INTVCC supplies enough current for the LT83203/LT83205’s circuitry. The voltage on INTVCC varies

between 2.6V and 3.4V when VIN is between 2.7V and 3.5V. Do not connect an external load to the INTVCC pin.

SET Pin Capacitor: Noise and Soft-Start

In addition to reducing output noise, using a SET pin bypass capacitor reduces the sensitivity to any parasitic

coupling of voltage spikes onto the SET pin. Note that any bypass capacitor leakage deteriorates the

LT83203/LT83205’s DC regulation. Capacitor leakage of even 100nA is a 0.1% DC error. Therefore, it is recommended

to use a good quality, low leakage ceramic capacitor.

Using a SET pin bypass capacitor also soft-starts the output and limits inrush current. Soft-starting the output

prevents a current surge on the input supply. The SET pin capacitor and resistor values set the ramp-up time of the

reference voltage, and the output voltage tracks this voltage. The SET pin resistor size is determined by the

application’s required output voltage; however, the capacitor size may be selected to achieve the desired ramp-up

time. It is important to consider that the size of the SET pin capacitor also plays a role in noise performance, which is

typically the more important factor in determining the size of this capacitor.

Ceramics are manufactured with a variety of dielectrics, each with a different behavior across temperature and

applied voltage. Care should be taken when selecting a ceramic capacitor for bypassing the SET pin, as this is a

critical component. An X7R (or better) ceramic capacitor is strongly recommended for its superior stability across

temperature and DC voltage bias. Additionally, larger case sizes are recommended for better DC bias and AC voltage

characteristics.