SC4806MLTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

Manufacturer

SC4806MLTRT

Multiple Function Double Ended PWM Controller

Semtech Corporation 

SC4806

POWER MANAGEMENT

Pin Descriptions

LUVLO (Pin 1):

RC (Pin 4):

Line undervoltage lockout pin. An external resistive di-

vider from the Input supply will program the undervoltage

lockout level. The external divider should be referenced

to the quiet analog ground. During the LUVLO, the driver

outputs are disabled. This pin can also function as an

Enable/Disable.

SS (Pin 2):

An internal 35µA current source charges the external

capacitor connected to this pin. This pin is connected to

one of the inputs of the PWM comparator. When the volt-

age on this SS pin increases, but less than 1/3 of the

feedback voltage VFB, the pulse width of OUTA and OUTB

gradually increases to achieve soft start. As the output

voltage increases and feedback loop enters regulation,

the PWM modulator is controlled by VFB. At normal opera-

tion, the voltage at SS pin is clamped at Vref.

When the Over Current is tripped, both OUTA and OUTB

are pulled low after a typical time delay (Typ. 100ns). At

the same time, the SS cap is gradually discharged via an

equivalent 10µA internal current source. When the volt-

age on SS pin is dropped below 0.8V, a new SS cycle is

initiated while the SS cap is charged with 35µA again.

The internal thermal protection circuit monitors the die

temperature. If the temperature exceeds 175oC, the con-

troller is completely shutdown. When the temperature is

dropped below 160oC, defined by the hysteresis, the con-

troller re-starts with soft start process.

SYNC (Pin 3):

SYNC is a positive edge triggered input with a threshold

set to 1.75V. In a single controller operation, SYNC could

be grounded or connected to an external synchroniza-

tion clock within the SYNC frequency range. In Bi-Phase

operation mode SYNC pins could be connected to the

Cosc (Timing Capacitors) of the other controller. This will

force an out-of-phase operation (see Application Infor-

mation part).

The oscillator programming pin. The oscillator should be

referenced to a stable reference voltage for an accu-

rate and stable frequency. Only two components are re-

quired to program the oscillator, a resistor (tied to Vref

and RC), and a capacitor (tied to the RC and GND). The

following formula can be used for a close approximation

of the oscillator frequency.

FOSC

≅

1

ROSCCTOT

where:

CTOT = COSC + CSC4806 + CCircuit

CSC4806 ≅ 22pF

The recommended range of timing resistors is between

10kohm and 200kohm and range of timing capacitors

is between 100pF and 1000pF. Timing resistors less

than 10kohm should be avoided.

Refer to layout guidelines in Application Information sec-

tion to achieve best results.

ILim (Pin 5):

The current signal from a sense resistor is applied to

peak current and overcurrent comparators through ILM

pin.

Under normal operation condition, the comparators are

not trigged. When the current signal sensed at ILM pin

exceeds the first threshold -- pulse-by-pulse current limit,

the corresponding on-time is terminated for the remain-

der of the switching cycle. In this case, the circuit output

voltage loses regulation even though it continues to pro-

vide full load current.

When the load current continuously increases and the

sensed signal at ILM pin reaches the second threshold -

- over current limit, the controller turns off both OUTA

and OUTB. At the same time, the SS cap is discharged

with equivalent 10uA current source. When the voltage

at SS pin is below 0.5V, the controller initiates re-start.

The pins Ramp and ILM are discharged by the internal

FETs at the end of each switching cycle.

 2006 Semtech Corp.

5

www.semtech.com

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