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 70µA, typical, of supply
current and can be placed in a 0.5µA 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
high. Other features include start-up at V
IN
= 0.9V and
an externally programmable soft start time.
3.1 Operating Mode
The TC110 is powered by the voltage present on the
V
DD
input. The applications circuits of Figure 3-1 and
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
supplied by V
IN
through the inductor and Schottky
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 maybetiedtotheinput
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 0.1µFforR
SS
and C
SS
,
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.
