TC115_13 Datasheet, PDF (5/14 Pages) Microchip Technology – PFM/PWM Step-Up DC/DC Converter

4.0 DETAILED DESCRIPTION
The TC115 is a combination PFM/PWM step-up
(boost) regulator. It is particularly useful in battery-pow-
ered 1, 2 and 3 cell applications where the required out-
put current is 140 mA 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 mod-
ulation (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.
4.1 Input Power and Sensing
The TC115 is powered from the PS input, which must
be connected to the regulated output, as shown in
Figure 5-1. 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.
4.2 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. While ultra fast diodes can be used, lower effi-
ciency will result due to their higher forward voltage
drop. Ordinary rectifiers should be avoided because of
their slow recovery characteristics.
TC115
4.3 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 regu-
lator is in shutdown. Care must be taken in
system design to assure the input supply
is isolated from the load during shutdown.
4.4 Behavior When V_IN is Greater
Than the Factory-Programmed
V_OUT Setting
The TC115 is designed to operate as a step-up
regulator only. As such, V_IN is assumed to always be
less than the factory-programmed V_OUT setting (V_R).
Operating the TC115 with V_IN > V_R causes regulating
action to be suspended (and corresponding supply
current reduction to 9 µA, typical) until V_IN is again less
than V_R. While regulating action is suspended, V_IN is
connected to V_OUT 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 V_IN/V_OUT leakage path is problematic.
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DS21361D-page 5