uP1728

4

uP1728-DS-F0000, Dec. 2012

www.upi-semi.com

The uP1728 is a high efficiency synchronous-rectified buck

converter with internal power switches. Fixed 1.2MHz

PWM operation allows possible smallest output ripple and

external component size. With high conversion efficiency

and small package, the uP1728 is ideally suitable for

portable devices and USB/PCIE-based interface cards

where PCB area is especially concerned. With internal

low R

3A output current over a wide input voltage range from

2.6V to 5.5V. The output voltage is adjustable from 0.6V

to VIN by a voltage divider. Other features include internal

soft-start, chip enable, over-voltage, under-voltage, over-

temperature and over-current protections. The uP1728 is

available in a space-saving WDFN3x3-10L package.

Input Supply Voltages, VIN & VDD

The uP1728 features separate power supply and ground

pins for power stages and control circuit, isolating the

control circuit from noise associated with the power

MOSFET switching. The VIN pins provide current to the

power stage. The supply voltage range is from 2.6V to

5.5V. The uP1728 draws pulsed current with sharp edges

from VIN each time the upper switch turns on, resulting in

voltage ripples and spikes at supply input. A minimum 10uF

ceramic capacitor with shortest PCB trace is highly

recommended for bypassing the supply input. The VDD

pin provides currents for the internal control circuit. A power

on reset (POR) continuously monitors the input supply

voltage. The POR level is typically 2.4V at VDD rising.

Use low pass filter R4 and C3 as shown in the Typical

Application Circuit to filter the input noise associated with

the power switching.

Chip Enable/Disable and Soft-Start

The uP1728 features an EN pin for enable/disable control

of the output voltage. Pulling the EN pin lower than 0.4V

shuts down the uP1728 and reduces its quiescent current

lower than 1uA. In the shutdown mode, both upper and

lower switches are turned off. Pulling EN pin higher than

1.5V enables the uP1728. Once the chip is enabled, the

VDD POR is granted. The internal soft-start capacitor

becomes charged and generates a linear ramping up

voltage across the capacitor. This voltage clamps the

voltage at the FB pin causing PWM pulse width to increase

slowly and in turn reduce the output surge current. The

internal 0.6V reference takes over the loop control once

the internal ramping-up voltage becomes higher than 0.6V.

PWM Operation

The uP1728 adopts slope-compensated, current mode

PWM control capable of achieving 100% duty cycle. During

normal operation, the uP1728 operates at PWM mode to

Functional Description

regulate output voltage by transferring the power to the

output voltage cycle by cycle at a constant 1.2MHz

frequency. The uP1728 turns on the upper switch at each

rising edge of the internal oscillator allowing the inductor

current to ramp up linearly. The switch remains on until

either the current-limit is tripped or the PWM comparator

turns off the switch for regulating output voltage. The upper

switch current is sensed, slope compensated and

compared with the error amplifier output COMP to

determine the adequate duty cycle. The VOUT pin senses

output feedback voltage from an external resistive divider.

When the load current increases, it causes a slight

decrease in the feedback voltage relative to the 0.6V

reference, which in turn, causes the error amplifier output

voltage to increase until the average inductor current

matches the new load current.

Low Dropout Mode

The uP1728 increases duty cycle to maintain output

voltage within its regulation as the supply input drops

gradually in the battery-powered applications. The uP1728

operates with 100% duty cycle and enters low dropout

mode as the supply input approaches the output voltage.

This maximizes the battery life.

Output Voltage Setting and Feedback Network

The output voltage can be set from VREF to VIN by a

voltage divider as:

REF

OUT

V

1

R

2

R

1

R

V

×

+

=

The internal VREF is 0.6V with 1% accuracy. In real

applications, a 22pF feed-forward ceramic capacitor is

recommended in parallel with R2 for better transient

response.

Current Limit Function

The uP1728 continuously monitors the inductor current

for current limit by sensing the voltage drop across the

upper switch when it turns on. When the inductor current

is higher than current limit threshold (6A typical), the

current limit function activates and forces the upper switch

turning off to limit inductor current cycle by cycle. If the

load continuously demands more current than what

uP1728 could provide, uP1728 can not regulate the output

voltage. Eventually under voltage protection will be

triggered and shuts down the uP1728 if VOUT is too low.

Under Voltage Protection

Under voltage Protection is triggered if the FB voltage is

lower than 0.26V. Once UVP is triggered , the uP1728

turn off high-side and low-side MOSFET. The under voltage

protection is latch-off function and can only be reset by