uP1784

4

uP1784-DS-F0000, Oct. 2013

www.upi-semi.com

The uP1784 contains two identical synchronous-rectified

buck converters running in out of phase operation. With

internal low R

delivering 1.3A peak output current over a wide input voltage

range from 2.6V to 5.5V. Fixed 1.5MHz operation allows

possible smallest output ripple and external component

size. With high conversion efficiency and small package,

the uP1784 is ideally suitable for portable devices and USB/

PCIE-based interface cards where PCB area is especially

concerned. The output voltage is adjustable from 0.6V to

V

start, chip enable, under-voltage, over-temperature and over-

current protections.

Input Supply Voltage, V

INX

VIN1 and VIN2 supply currents to the internal control circuits

and supply currents to the output voltages. The supply

voltage range is from 2.6V to 5.5V. A power on reset (POR)

continuously monitors the input supply voltage. The POR

level is typically 2.5V at V

The uP1784 draws pulsed current with sharp edges each

time the upper switch turns on, resulting in voltage ripples

and spikes at supply input. A minimum 4.7uF ceramic

capacitor with shortest PCB trace is highly recommended

for bypassing the supply input.

Chip Enable/Disable and Soft Start

Pulling EN pin lower than 0.4V shuts down the uP1784

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 uP1784 and

initiates the soft start cycle. The uP1784 limits the in-rush

current at start-up. This prevents unwanted shutdown

otherwise may be triggered by voltage drop due to large

inrush current.

PWM Operation

The uP1784 adopts slope-compensated, current mode

PWM control capable of achieving 100% duty cycle. During

normal operation, the uP1784 operates at PWM mode to

regulate output voltage by transferring the power to the

output voltage cycle by cycle at a constant 1.5MHz

frequency. The uP1784 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.

The lower switch turns on with optimal deadtime and picks

up the inductor current after the upper switch turns off,

allowing the inductor current to ramp down linearly. The

switch remains on until the next rising edge of oscillator

turns on the upper switch. The uP1784 regulates the output

voltage by controlling the ramp up/down duty cycle of

inductor current. The high frequency switching ripple is

easily smoothed by the output filter.

A Pseudo Diode Emulator monitors inductor current by

sensing voltage drop across the lower switch when it turns

on. The lower switch acts a free wheel diode in an

asynchronous buck converter when the inductor current is

lower than 80mA. This allows the converter operating in

discontinuous conduction mode (DCM) and reduces

conduction loss and increase power conversion efficiency

at light condition.

PSM Operation

A finite minimum on-time for both upper and lower switches

is implemented for normal operation. Consequently, the

converter will enter pulse-skipping mode (PSM) during

extreme light load condition or when modulation index (V

V

is reduced. This could reduce switching loss and further

increase power conversion efficiency.

Low Dropout Mode

The uP1784 increases duty cycle to maintain output voltage

within its regulation as the supply input drops gradually in

the battery-powered applications. The uP1784 operates with

100% duty cycle and enters low dropout mode as the supply

input approaches the output voltage. This maximizes the

battery life.

Current Limit Function

The uP1784 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 (1.4A typical), the current limit

function activates and forces the upper switch to turn off to

limit inductor current cycle by cycle. If the load continuously

demands more current than what uP1784 could provide,

uP1784 can not regulate the output voltage. Eventually

under voltage protection will be triggered and shuts down

the uP1784 if V

Functional Description