200mA, Negative-Output, Low-Dropout

Linear Regulator in SOT23

8

_______________________________________________________________________________________

Operating Region and Power Dissipation

Maximum power dissipation of the MAX1735 depends

on the thermal resistance of the case and the circuit

board, the temperature difference between the die

junction and ambient air, and the rate of air flow (see

also Thermal Overload Protection). The maximum

power that can be dissipated by the device is:

where the numerator expresses the temperature differ-

ence between the maximum allowed die junction

(+150°C) and the surrounding air,

is the thermal resistance of the package, and

to ambient) is the thermal resistance from the package

through the PC board, traces, and other material to the

surrounding air. The former is a characteristic solely of

the device in its package, and the latter is completely

defined by PC board layout and airflow. It is important to

note that the ability to dissipate power is as much a func-

tion of the PC board layout and air flow as the packaged

part itself. Hence, a manufacturer can reliably provide a

value for

total thermal resistance

the thermal characteristics of the application circuit.

Figure 4 shows the estimated allowable power dissipa-

tion for a MAX1735 mounted on a typical PC board at

ambient temperatures of +50°C, +70°C, and +85°C.

Figure 4 shows the maximum continuous output current

for a particular input-to-output voltage differential, for

selected ambient temperatures. The working principle is

that the SOT23-5 package is small enough that in a typi-

cal application circuit at room temperature, the package

cannot dissipate enough power to allow -6.5V to be reg-

ulated to -1.25V at -200mA output (more than 1200mW).

As ambient temperature falls, the available power dissi-

pation increases to allow for a greater operating region.

The equation for the family of curves follows:

where

is the absolute maximum rated power dissipation at

+70°C for the SOT23-5, and

approximate junction-to-ambient thermal resistance of

the SOT23-5 in a typical application.

A key to reducing

ductivity to the PC board, is to provide large PC board

pads and traces for IN.

__________Applications Information

Capacitor Selection and

Regulator Stability

Capacitors are required at the input and output of the

MAX1735. Connect a 1µF or greater capacitor between

IN and GND. This input capacitor serves only to lower

the source impedance of the input supply in transient

conditions; a smaller value can be used when the regu-

lator is powered from a low-impedance source, such as

another regulated supply or low-impedance batteries.

For output voltages between -2.5V and -5.5V, connect

a 1µF or greater capacitor between OUT and GND. For

voltages between -1.25V and -2.5V, use a 2.2µF or

greater output capacitor. The maximum value of the

output capacitor to guarantee stability is 10µF.

The output capacitor’s value and equivalent series

resistance (ESR) affect stability and output noise. To

ensure stability and optimum transient response, output

capacitor ESR should be 0.1

Ω or less for output volt-

ages from -1.25V to -2.45V and 0.2

Ω or less for output

voltages between -2.5V and -5.5V. Inexpensive sur-

face-mount ceramic capacitors typically have very-low

ESR and are commonly available in values up to 10µF.

Other low-ESR capacitors, such as surface-mount tan-

talum, may also be used. Do not use low-cost alu-

minum electrolytic capacitors due to their large size

and relatively high ESR. Lastly, make sure the input and

output capacitors are as close to the IC as possible to

minimize the impact of PC board trace impedance.

||

||

I

P

T

VV

OUT

MAX

A

JA

OUT

IN

=

−

−

−

70

θ

P

TT

TT

MAX

JMAX

A

JC

CA

JMAX

A

JA

=

−

+

=

−

θθ

θ

250

150

100

50

0

03

12

4

5

6

MAXIMUM OUTPUT CURRENT

vs. INPUT-OUTPUT VOLTAGE DIFFERENTIAL

INPUT-OUTPUT VOLTAGE DIFFERENTIAL (V)

AT MAXIMUM

JUNCTION TEMP

°C)

200

MAXIMUM CONTINUOUS CURRENT

T

°C

T

°C

T

°C

Figure 4. Output Current and In-Out Voltage Differential

Operating Region Bounded by Available Power Dissipation at

Selected Ambient Temperatures