TC125 datasheet(4/12 Pages) MICROCHIP

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TC125/TC126

DS21372B-page 4

3.0

DETAILED DESCRIPTION

The TC125/126 are PFM step-up DC/DC regulators for

use in systems operating from two or more cells or in

low voltage, line powered applications. Because Pulse

Frequency Modulation (PFM) is used, the TC125/126

switching frequency (and therefore supply current) is

minimized at low output loads. This is especially

important in battery operated applications (such as

pagers) that operate in standby mode most of the time.

For example, a TC125/126 with a 3V output and no

load will consume a maximum supply current of only

µA versus a supply current of 40µA maximum when

IOUT = 30mA. Both devices require only an external

inductor, diode and capacitor to implement a complete

DC/DC converter.

The TC125 is recommended for applications requiring

shutdown mode as a means of reducing system supply

current. The TC125 is powered from the PS input,

which must be connected to the regulated output as

shown in Figure 3-1. 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.

The TC126 (Figure 3-2) is recommended for all appli-

cations not requiring shutdown mode. It has separate

VDD and SENSE inputs, allowing it to be powered from

any source of 2.2V to 10V in the system. The VDD input

of the TC126 may be connected to the VIN,VOUT,oran

external DC voltage. Lower values of VDD result in

lower supply current, but lower efficiency due to higher

switch ON resistance. Higher VDD values increase

supply

current,

but drive the internal switching

transistor harder (lowering RDSON), thereby increasing

efficiency.

3.1

Low Power Shutdown Mode

The TC125 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. Because the TC125 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.

3.2

Behavior When VIN is Greater

Than the Factory-Programmed

OUT Setting

The TC125 and TC126 are designed to operate as

step-up regulators only. As such, VIN is assumed to

always be less than the factory-programmed output

voltage setting (VR). Operating the TC125/126 with

VIN >VR causes regulating action to be suspended

(and corresponding supply current reduction) until VIN

is again less than VR. While regulating action is

suspended, VIN is connected to the output voltage node

through the series combination of the inductor and

Schottky diode. Again, care must be taken to add the

appropriate isolation (MOSFET series switch or post

LDO with shutdown) during system design if this VIN/

VOUT leakage path is problematic.

FIGURE 3-1:

TYPICAL TC125 CIRCUIT

FIGURE 3-2:

TYPICAL TC126 CIRCUIT

100

µH Sumida CD54

MA735

Shutdown

Control

TC125

SHDN

GND

1.5V

OFF

VOUT

3.3V @40mA

µF/16V

Tantalum

TC126

SENSE

VDD

GND

3.3V

Line

Supply

100

µH Sumida CD54

MA735

VOUT

5V @80mA

µF/16V

Tantalum

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