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TEA1541

SMPS control IC with synchronization function

Philips Electronics 

Philips Semiconductors

SMPS control IC with

synchronization function

Product specification

TEA1541

6.6 Minimum and maximum ‘on-time’

The minimum on-time of the converter is not limited by the

leading edge blanking time, and therefore can be zero.

The IC limits the maximum on-time to f--s---m-1---p---s-

where fsmps is the converter switching frequency in either

synchronized or unsynchronized mode. If the system

requires a longer on-time, a fault condition is assumed, for

example, if CVIN is removed, the IC will stop switching and

enter the safe restart mode.

6.7 Overvoltage protection

The TEA1541 allows OVP to be set accurately.

The flyback converter output voltage is accurately

represented by the voltage across the auxiliary winding.

The auxiliary winding voltage is monitored by the current

flowing into pin DEM during the demagnetizing cycle of the

transformer. The inevitable voltage spikes at pin DEM are

reduced using an internal filter.

If the output voltage causes the current into pin DEM to

exceed the OVP level lOVP(DEM), the OVP circuit turns off

the power MOSFET. The controller then waits until the

VCC(UVLO) condition is reached. This is followed by a safe

restart cycle, before switching recommences.

This process is repeated until the OVP condition ends.

The output voltage at which OVP activates, Vo(ovp) is set by

the value of resistor, RDEM, (see Fig.8) using the equation:

Vo(ovp) = N---N--a---us--x- × (IOVP(DEM) × RDEM + Vclamp(DEM)(pos))

where N is the number of turns on the transformer

windings; Vclamp(DEM)(pos) is the positive clamp voltage on

pin DEM; reference current IOVP(DEM) is set internally.

6.8 Overcurrent protection and overpower

protection

The current in the transformer primary is measured

accurately by the internal cycle-by-cycle source current

limit circuit using the external sense resistor Rsense.

The accuracy of the current limit circuit allows the

transformer core to have a minimum specification for the

output power required. The OCP circuit limits the ‘sense’

voltage to an internal level, and is activated after the

leading edge blanking period, tleb generated by the

Leading Edge Blanking circuit (LEB shown in Fig.2).

Leading edge blanking is required to inhibit OCP for a

short period when the power MOSFET turns on.

This ensures that the MOSFET is not turned off

prematurely due to the false sensing of an overcurrent

condition caused by current spikes produced by the

discharge of primary-side snubber and parasitic

capacitances.

The OCP level is adjusted proportionally to the switching

frequency such that the product of (Ipeak)2 × frequency

stays constant. This arrangement also implements OPP,

ensuring that the maximum output power is independent of

the switching frequency, otherwise the output power would

increase in direct proportion to the switching frequency.

6.9 Soft start

The soft start function allows the transformer peak current

to slowly increase at every start-up and restart, to prevent

transformer rattle.

The soft start function requires a resistor RSS and

capacitor CSS to be connected between pin Isense and the

sense resistor Rsense (see Fig.7). CSS is charged by an

internal current source ISS to V = ISS × RSS, to a maximum

of approximately 0.5 V.

handbook, halfpage

0.5 V

ISS

start-up

5 Isense RSS

Vocp

CSS

Rsense

MBL503

Fig.7 Soft start.

The rate at which the primary current increases can be

adjusted by changing the values of RSS and CSS to change

the circuit time constant: τ = RSS × CSS

The maximum primary current is calculated by the

equation: Iprimary(max) = V-----s--e---n---s---e---(-m----a--R-x---)s--e-–--n--s(---eI--S---S----×-----R-----S---S---)-

2003 Aug 11

9

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