TC1313 Datasheet, PDF (14/28 Pages) Microchip Technology – 500 mA Synchronous Buck Regulator, + 300 mA LDO

TC1313
4.0 DETAILED DESCRIPTION
4.1 Device Overview
The TC1313 combines a 500 mA synchronous buck
regulator with a 300 mA LDO. This unique combination
provides a small, low-cost solution for applications that
require two or more voltage rails. The buck regulator
can deliver high-output current over a wide range of
input-to-output voltage ratios while maintaining high
efficiency. This is typically used for the lower-voltage,
higher-current processor core. The LDO is a minimal
parts-count solution (single-output capacitor), providing
a regulated voltage for an auxiliary rail. The typical LDO
dropout voltage (137 mV @ 200 mA) allows the use of
very low input-to-output LDO differential voltages,
minimizing the power loss internal to the LDO pass
transistor. Integrated features include independent
shutdown inputs, UVLO, overcurrent and
overtemperature shutdown.
4.2 Synchronous Buck Regulator
The synchronous buck regulator is capable of supply-
ing a 500 mA continuous output current over a wide
range of input and output voltages. The output voltage
range is from 0.8V (min) to 4.5V (max). The regulator
operates in three different modes and automatically
selects the most efficient mode of operation. During
heavy load conditions, the TC1313 buck converter
operates at a high, fixed frequency (2.0 MHz) using
current mode control. This minimizes output ripple and
noise (less than 8 mV peak-to-peak ripple) while main-
taining high efficiency (typically > 90%). For standby or
light-load applications, the buck regulator will automat-
ically switch to a power-saving Pulse Frequency
Modulation (PFM) mode. This minimizes the quiescent
current draw on the battery while keeping the buck
output voltage in regulation. The typical buck PFM
mode current is 38 µA. The buck regulator is capable of
operating at 100% duty cycle, minimizing the voltage
drop from input to output for wide-input, battery-
powered applications. For fixed-output voltage applica-
tions, the feedback divider and control loop compensa-
tion components are integrated, eliminating the need
for external components. The buck regulator output is
protected against overcurrent, short circuit and over-
temperature. While shut down, the synchronous buck
N-channel and P-channel switches are off, so the L_X
pin is in a high-impedance state (this allows for
connecting a source on the output of the buck regulator
as long as its voltage does not exceed the input
voltage).
4.2.1 FIXED-FREQUENCY PWM MODE
While operating in Pulse Width Modulation (PWM)
mode, the TC1313 buck regulator switches at a fixed
2.0 MHz frequency. The PWM mode is suited for higher
load current operation, maintaining low output noise
and high conversion efficiency. PFM to PWM mode
transition is initiated for any of the following conditions.
• Continuous inductor current is sensed
• Inductor peak current exceeds 100 mA
• The buck regulator output voltage has dropped
out of regulation (step load has occurred)
The typical PFM-to-PWM threshold is 80 mA.
4.2.2 PFM MODE
PFM mode is entered when the output load on the buck
regulator is very light. Once detected, the converter
enters the PFM mode automatically and begins to skip
pulses to minimize unnecessary quiescent current
draw by reducing the number of switching cycles per
second. The typical quiescent current for the switching
regulator is less than 38 µA. The transition from PWM
to PFM mode occurs when discontinuous inductor
current is sensed, or the peak inductor current is less
than 60 mA (typ.). The typical PWM to PFM mode
threshold is 30 mA. For low input-to-output differential
voltages, the PWM to PFM mode threshold can be low
due to the lack of ripple current. It is recommended that
V_IN1 be one volt greater than V_OUT1 for PWM to PFM
transitions.
4.3 Low-Dropout Regulator (LDO)
The LDO output is a 300 mA low-dropout linear regula-
tor that provides a regulated output voltage with a
single 1 µF external capacitor. The output voltage is
available in fixed options only, ranging from 1.5V to
3.3V. The LDO is stable using ceramic output capaci-
tors that inherently provide lower output noise and
reduce the size and cost of the regulator solution. The
quiescent current consumed by the LDO output is
typically less than 43.7 µA, with a typical dropout volt-
age of 137 mV at 200 mA. The LDO output is protected
against overcurrent and overtemperature. While oper-
ating in Dropout mode, the LDO quiescent current will
increase, minimizing the necessary voltage differential
needed for the LDO output to maintain regulation. The
LDO output is protected against overcurrent and
overtemperature.
DS21974A-page 14
© 2005 Microchip Technology Inc.