SP6139EU-L datasheet(6/16 Pages) EXAR

Part #	SP6139EU-L

Description	Wide Input, 1.3MHz Synchronous PWM Step Down Controller

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Manufacturer	EXAR [Exar Corporation]

Direct Link	http://www.exar.com

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THEORY OF OPERATION

GENERAL OVERVIEW

The SP6139 is a fixed frequency, voltage

mode, synchronous PWM controller optimized

for high efficiency. The part has been designed

to be especially attractive for split plane

applications utilizing 5V to power the controller

and 3V to 15V for step down conversion. The

heart of the SP6139 is a wide bandwidth

transconductance

amplifier

designed

to

accommodate

Type

II

and

Type

III

compensation schemes. A precision 0.8V

reference present on the positive terminal of

the error amplifier permits the programming

of the output voltage down to 0.8V via the VFB

pin. The output of the error amplifier, COMP,

compared to a 1.1V peak-to-peak ramp is

responsible for trailing edge PWM control. This

voltage ramp and PWM control logic are

governed by the internal oscillator that

accurately sets the PWM frequency to 300kHz.

The SP6139 contains two unique control

features that are very powerful in distributed

applications. First, Non-synchronous driver

control is enabled during start up to prohibit

the low side NFET from pulling down the

output until the high side NFET has attempted

to turn on. Second, a 100% duty cycle timeout

ensures that the low side NFET is periodically

enhanced during extended periods at 100%

duty cycle. This guarantees the synchronized

refreshing of the BST capacitor during very

large duty ratios. The SP6139 also contains a

number of valuable protection features. A

programmable input (VIN) UVLO allows a user

to set the exact value at which the conversion

voltage is at a safe point to begin down

conversion, and an internal VCC UVLO ensures

that the controller itself has enough voltage to

properly operate. Other protection features

include thermal shutdown and short-circuit

detection. In the event that either a thermal,

short-circuit, or UVLO fault is detected, the

SP6139 is forced into an idle state where the

output drivers are held off for a finite period

before a re-start is attempted.

SOFT START

“Soft Start” is achieved when a power

converter ramps up the output voltage while

controlling the magnitude of the input supply

source current. In a modern step down

converter, ramping up the positive terminal of

the error amplifier controls soft start. As a

result, excess source current can be defined as

the current required to charge the output

capacitor.

IVIN = COUT * DVOUT / DTSoft-start

The SP6139 provides the user with the option

to program the soft start rate by tying a

capacitor from the SS pin to GND. The

selection of this capacitor is based on the

10uA pull up current present at the SS pin and

the 0.8V reference voltage. Therefore, the

excess source can be redefined as:

IVIN = COUT * DVOUT *10μA / (CSS * 0.8V)

UNDER VOLTAGE LOCK OUT (UVLO)

The SP6139 contains two separate UVLO

comparators to monitor the bias (VCC) and

conversion (VIN) voltages independently. The

VCC UVLO threshold is internally set to 4.25V,

whereas

the

VIN

UVLO

threshold

is

programmable through the UVIN pin. When

the UVIN pin is greater than 2.5V, the SP6139

is permitted to start up pending the removal of

all other faults. Both the VCC and VIN UVLO

comparators

have

been

designed

with

hysteresis to prevent noise from resetting a

fault.

THERMAL AND SHORT-CIRCUIT PROTECTION

Because the SP6139 is designed to drive large

NFETs running at high current, there is a

chance that either the controller or power

converter will become too hot. Therefore, an

internal thermal shutdown (145°C) has been

included to prevent the IC from malfunctioning

at extreme temperatures.

A short-circuit detection comparator has also

been included in the SP6139 to protect against

the accidental short or sever build up of

current at the output of the power converter.

This comparator constantly monitors the

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