TC115 データシートの表示（PDF） - Microchip Technology

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TC115

300mA CMOS LDO

Microchip Technology 

TC115

3.0 DETAILED DESCRIPTION

The TC115 is a combination PFM/PWM step-up

(boost) regulator. It is particularly useful in 1, 2 and 3

cell applications where the required output current is

140mA or less, and size/cost issues are a concern. The

device operates in PWM mode when the output load is

sufficient to demand a 10% (or greater) duty cycle.

While in PWM mode, the TC115 behaves as any other

PWM switching regulator, to a maximum duty cycle of

92%. At low output loads (i.e., output loads requiring

< 10% duty cycle to support); the TC115 automatically

switches to pulse frequency modulation (PFM)

operating mode with a fixed duty cycle of 25%, max,

(17%, typical). While in PFM mode, the inductor is

modulated with individual fixed width pulses only as

needed to maintain output voltage. This action reduces

supply current, thereby improving power efficiency at

low output loads.

3.1 Input Power and Sensing

The TC115 is powered from the PS input, which must

be connected to the regulated output as shown in

Figure 4-1. PS also senses output voltage for closed-

loop regulation. Start-up current is furnished through

the inductor when input voltage is initially applied. This

action starts the oscillator, causing the voltage at the

PS input to rise, bootstrapping the regulator into full

operation.

3.2 Output Diode

For best results, use a Schottky diode such as the

MA735, 1N5817, EC10 or equivalent. Connect the

diode between the PS and LX pins as close to the IC as

possible. Do not use ordinary rectifier diodes since the

higher forward voltages reduce efficiency.

3.3 Low Power Shutdown Mode

The TC115 enters a low power shutdown mode when

SHDN is brought low. While in shutdown, the oscillator

is disabled and the internal switch is shut off. Normal

regulator operation resumes when SHDN is brought

high. SHDN may be tied to the input supply if not used.

Note:

Because the TC115 uses an external diode,

a leakage path between the input voltage

and the output node (through the inductor

and diode) exists while the regulator is in

shutdown. Care must be taken in system

design to assure the input supply is isolated

from the load during shutdown.

3.4 Behavior When VIN is Greater

Than the Factory-Programmed

VOUT Setting

The TC115 is designed to operate as a step-up

regulator only. As such, VIN is assumed to always be

less than the factory-programmed VOUT setting (VR).

Operating the TC115 with VIN > VR causes regulating

action to be suspended (and corresponding supply

current reduction to 9µA, typical) until VIN is again less

than VR. While regulating action is suspended, VIN is

connected to VOUT through the series combination of

the inductor and Schottky diode. Care must be taken to

add the appropriate isolation (MOSFET output switch

or post LDO with shutdown) during system design if

this VIN/VOUT leakage path is problematic.

DS21361B-page 4

© 2002 Microchip Technology Inc.

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