SC4808B-1MSTRT Datasheet SC4808B-1MSTRT | P13-P15

13 2006 Semtech Corp. www.semtech.com

SC4808B-1

POWER MANAGEMENT

It should be noted that if high speed/high current drivers

such as the SC1301 are used, careful layout guide lines

must be followed in order to minimize stray inductance,

which might cause negative voltages at the output of the

drivers. This negative voltage can be clamped to a reason-

able level by placing a small Schottky diode directly at the

output of the driver as shown below.

C34

0.1u

VCC

SC1301A

1 4

C23

0.1u

SC1301A

1 4

VCC

R28

2.2u,16V

C26

D_A

0.1u,25V

C33

R33

10k

SYNC

SC4808

101

REF

LUVLOSYNC

GND

OUTB

OUTA

VCC

D_B

Gate_A

Gate_B

R23

56.2k

Vin

OVER CURRENT

Two levels of over current protection are provided by the

SC4808B-1. The current information is sensed at the CS

pin and compared to a peak current limit level of 525mV.

If the 525mV limit is exceeded, the OUTA and OUTB pulse

widths and duty cycle is reduced until the CS pin reaches a

second threshold of 950mV. At that point, the OUTA and

OUTB are disabled, and after a delay of 140µs, the inter-

nal softstart sequence is started. After the softstart dura-

tion (see page 18 for calculation of softstart time), normal

operation is achieved, unless the over current condition is

still present.

Application Information (Cont.)

14 2006 Semtech Corp. www.semtech.com

SC4808B-1

POWER MANAGEMENT

D<50%

Instability in current mode operation

due to Duty cycle >50%

: Inductor current

: Small Inductor current perturbation

Note: After afew cycles the perturbation

disappears and stable operation returns.

D=50%

Note: After afew cycles the perturbation

is still present, although this will cause

jitter, but there is no instability.

D>50%

Note: After afew cycles the perturbation

becomes larger, and causes instability.

Error

Time

Error

Time

Error

Time

SLOPE COMPENSATION (Current or Voltage mode of operation)

In applications where a current mode control is used for regulation, the peak inductor current information is used to

produce the average output current. If a small perturbation due to changes in supply voltage or noise pick up is gener-

ated, instability may occur if the duty cycle is >50%.

This phenomenon is graphically shown below. The inductor current and disturbed inductor current are shown for three

different duty cycles conditions.

The top wave form shows the applications where the duty cycle D is less than 50%. As shown, even if an error is

introduced, after only a few cycles the error converges to zero.

The second wave form shows the case where D = 50%. Under this condition, even though the error does not completely

disappear, it stays constant and is not getting larger. This will be seen as jitter at the inductor voltage.

The bottom wave form shows D>50%. As shown, a very small error results in a much larger error only after a few cycles.

This will cause instability in the converter and the average output inductor current. The output load will not be able to be

kept in regulation.

Application Information (Cont.)

15 2006 Semtech Corp. www.semtech.com

SC4808B-1

POWER MANAGEMENT

The instability can be corrected by modification of the peak

current information slope. One of the methods to alter the

peak current information is to add a positive going ramp to

the output of the current sensing circuitry.

The SC4808B-1 achieves this by using an internal slope

compensation circuit. The oscillator ramp is internally buff-

ered and an internal 25kOhms resistor in conjunction with

an external resistor at the CS pin will program the level of

slope compensation.

Rosc

15k

10k

200p

Cosc

RSlope Comp

2.2u,16V

82p

Cfilter

REF

Rsense

SC4808

REF

SYNC

GND

N = 100

25k

Current Transformer

RSlope Comp value will determine the Mode of operation (Voltage or Current)

The Peak current information is sensed and the result is

realistically summed to the buffered oscillator ramp, as

shown above. The value of the external resistor R

slope comp

will determine the percentage of the slope compensation.

As the value for R

slope comp

is reduced, the current informa-

tion becomes more dominant and the mode of operation

becomes more current mode. At the same time the slope

of the current information is modified to provide the slope

compensation.

Application Information (Cont.)

If the R

slope comp

is increased, the internal ramp becomes the

dominant signal and more voltage mode of operation is

achieved. As it can be calculated from the second formula

below, a 100% voltage mode operation can be achieved by

choosing R

slope comp

to be greater

than 6.25K ohms. Also if a

100% current mode of operation is required, R

slope comp

reduced to zero and the contribution from the internal ramp

is completely eliminated.

()

ernalintsenseComp_slope

senseComp_slope

Ramp

RRR

Comp_Slope%

()













×−

•≅

Comp_slope%2.01

Comp_slope%2.0

ernalintexternal

Next page illustrates how the buffered oscillator ramp is

used to modify the sensed inductor current.

It should be noted that in order for the slope compensa-

tion to be effective, the current sensed signal slope should

be at least 50% less steeper than the oscillator positive

ramp slope. The slope will include the magnetizing current

of the transformer and the inductor output current in iso-

lated applications. In non-isolated applications, the slope

will only include the inductor output current.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P28

SC4808B-1MSTRT

Mfr. #:

Buy SC4808B-1MSTRT

Manufacturer:

Semtech

Description:

IC REG CTRLR MULT TOP 10MSOP

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet

SC4808B-1MSTRT