TC1303A Datasheet, PDF(20/36 Page) Microchip Technology – 500 mA Synchronous Buck Regulator, + 300 mA LDO with Power-Good Output

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TC1303A Datasheet, PDF (20/36 Pages) Microchip Technology – 500 mA Synchronous Buck Regulator, + 300 mA LDO with Power-Good Output

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TC1303A/TC1303B/TC1303C/TC1304

4.0 DETAILED DESCRIPTION

4.1 Device Overview

The TC1303/TC1304 combines a 500 mA synchro-

nous buck regulator with a 300 mA LDO and a power-

good output. 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, high-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. A

power-good output is provided, indicating that the buck

regulator output, the LDO output or both outputs are in

regulation. Additional features include independent

shutdown inputs (TC1303), UVLO, output voltage

sequencing

(TC1304),

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, automatically select-

ing the most efficient mode of operation. During heavy

load conditions, the TC1303/TC1304 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 ÂµA. 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 LX

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 TC1303/TC1304 buck regulator switches at

a fixed, 2.0 MHz frequency. The PWM mode is suited

for higher load current operation, maintaining low out-

put noise and high conversion efficiency. PFM-to-PWM

mode transition is initiated for any of the following

conditions:

â¢ Continuous inductor current is sensed

â¢ Inductor peak current exceeds 100 mA

â¢ 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 35 ÂµA. 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

VIN1 be one volt greater than VOUT1 for PWM-to-PFM

transitions.

4.3 Low Drop Out Regulator (LDO)

The LDO output is a 300 mA low-dropout linear regula-

tor that provides a regulated output voltage with a

single 1 ÂµF 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 40 ÂµA, with a typical dropout voltage

of 137 mV at 200 mA. While operating in Dropout

mode, the LDO quiescent current will increase, mini-

mizing the necessary voltage differential needed for the

LDO output to maintain regulation. The LDO output is

protected against overcurrent and overtemperature

conditions.

DS21949B-page 20

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