TC110

DS21355B-page 4

© 2002 Microchip Technology Inc.

3.0

DETAILED DESCRIPTION

The TC110 is a PFM/PWM step-up DC/DC controller

for use in systems operating from two or more cells, or

in low voltage, line-powered applications. It uses PWM

as the primary modulation scheme, but automatically

converts to PFM at output duty cycles less than

approximately 25%. The conversion to PFM provides

reduced supply current, and therefore higher operating

efficiency at low loads. The TC110 uses an external

switching transistor, allowing construction of switching

regulators with maximum output currents of 300mA.

The TC110 consumes only 70

µA, typical, of supply

current and can be placed in a 0.5

µA shutdown mode

by bringing the shutdown input (SHDN) low. The

regulator remains disabled while in shutdown mode,

and normal operation resumes when SHDN is brought

an externally programmable soft start time.

3.1

Operating Mode

The TC110 is powered by the voltage present on the

Figure 3-2 show operation in the bootstrapped and

non-bootstrapped modes. In bootstrapped mode, the

TC110 is powered from the output (start-up voltage is

diode while Q1 is off). In bootstrapped mode, the

switching transistor is turned on harder because its

gate voltage is higher (due to the boost action of the

regulator), resulting in higher output current capacity.

The TC110 is powered from the input supply in the non-

bootstrapped mode. In this mode, the supply current to

the TC110 is minimized. However, the drive applied to

the gate of the switching transistor swings from the

input supply level to ground, so the transistor’s ON

resistance increases at low input voltages. Overall

efficiency is increased since supply current is reduced,

and less energy is consumed charging and discharging

the gate of the MOSFET. While the TC110 is guaran-

teed to start up at 0.9V the device performs to

specifications at 2.0V and higher.

3.2

Low Power Shutdown Mode

The TC110 enters a low power shutdown mode when

SHDN is brought low. While in shutdown, the oscillator

is disabled and the output switch (internal or external)

is shut off. Normal regulator operation resumes when

SHDN is brought high. SHDN maybetiedtothe input

supply if not used.

Note:

Because the TC110 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.

3.3

Soft Start

Soft start allows the output voltage to gradually ramp

from 0V to rated output value during start-up. This

action minimizes (or eliminates) overshoot, and in

general,

reduces

stress

on

circuit

components.

Figure 3-3 shows the circuit required to implement soft

start (values of 470K and 0.1

,

respectively, are adequate for most applications).

3.4

Input Bypass Capacitors

Using an input bypass capacitor reduces peak current

transients drawn from the input supply and reduces the

switching noise generated by the regulator. The source

impedance of the input supply determines the size of

the capacitor that should be used.