VIPER53ESP-E Datasheet VIPER53EDIP-E, VIPER53ESP-E, VIPER53ESPTR-E | P13-P15

VIPer53EDIP - E / VIPer53ESP - E High Voltage Start-up Current Source

DocRev1 13/31

8 High Voltage Start-up Current Source

An integrated high voltage current source provides a bias current from the DRAIN pin during

the start-up phase. This current is partially absorbed by internal control circuits in standby

mode with reduced consumption, and also supplies the external capacitor connected to the

pin. As soon as the voltage on this pin reaches the high voltage threshold V

DDon

of the

UVLO logic, the device turns into active mode and starts switching. The start-up current

generator is switched off, and the converter should normally provide the needed current on

the V

pin through the auxiliary winding of the transformer, as shown on Figure 3 on

page 8.

The external capacitor C

VDD

on the V

pin must be sized according to the time needed by

the converter to start-up, when the device starts switching. This time tss depends on many

parameters, including transformer design, output capacitors, soft start feature, and

compensation network implemented on the COMP pin and possible secondary feedback

circuit. The following formula can be used for defining the minimum capacitor needed:

Figure 9 on page 22 shows a typical start-up event. V

starts from 0V with a charging

current I

DDch1

at about 9 mA. When about V

DDoff

is reached, the charging current is reduced

down to I

DDch2

which is about 0.6mA. This lower current leads to a slope change on the V

rise. Device starts switching for V

equal to V

DDon

, and the auxiliary winding delivers some

energy to V

capacitor after the start-up time tss.

The charging current change at V

DDoff

allows a fast complete start-up time t

SDU

, and

maintains a low restart duty cycle. This is especially useful for short circuits and overloads

conditions, as described in the following section.

VDD

DD1

tss⋅

DDhyst

------------------------>

High Voltage Start-up Current Source VIPer53EDIP - E / VIPer53ESP - E

14/31 DocRev1

Figure 5. Start-up Waveforms

DD1

tSS

DDch2

DDch1

DDsd

DDst

DDreg

tSU

VIPer53EDIP - E / VIPer53ESP - E Short-Circuit and Overload Protection

DocRev1 15/31

9 Short-Circuit and Overload Protection

A V

COMPovl

threshold of about 4.4V has been implemented on the COMP pin. When V

COMP

goes above this level, the capacitor connected on the TOVL pin begins to charge. When

reaching typically V

OVLth

(4V), the internal MOSFET driver is disabled and the device stops

switching. This state is latched because of to the regulation loop which maintains the COMP

pin voltage above the V

COMPovl

threshold. Since the V

pin does not receive any more

energy from the auxiliary winding, its voltage drops down until it reaches V

off and the

device is reset, recharging the V

capacitor for a new restart cycle. Note: If VCOMP drops

below the VCOMPovl threshold for any reason during the VDD drop, the device resumes

switching immediately.

The device enters an endless restart sequence if the overload or short circuit condition is

maintained. The restart duty cycle D

RST

is defined as the time ratio for which the device tries

to restart, thus delivering its full power capability to the output. In order to keep the whole

converter in a safe state during this event, D

RST

must be kept as low as possible, without

compromising the real start-up of the converter. A typical value of about 10% is generally

sufficient. For this purpose, both V

and TOVL capacitors can be used to satisfy the

following conditions:

Refer to the previous start-up section for the definition of tss, and C

VDD

must also be

checked against the limit given in this section. The maximum value of the two calculus will

be adopted.

All this behavior can be observed on Figure 2 on page 7. In Figure 7 on page 20 the value of

the drain current Id for V

COMP

= V

COMPovl

is shown. The corresponding parameter I

Dmax

the drain current to take into account for design purposes. Since I

Dmax

represents the

maximum value for which the overload protection is not triggered, it defines the power

capability of the power supply.

OVL

12.5 10

6–

tss⋅⋅>

VDD

810

4 1

RST

-------------- 1–

OVL

DDch2

⋅

DDhyst

------------------------------------⋅⋅ ⋅>

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P31

VIPER53ESP-E

Mfr. #:

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Manufacturer:

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AC/DC Converters OFF line Primary Swtch

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