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TEA1507

GreenChipII SMPS control IC

Philips Electronics 

Philips Semiconductors

GreenChip™II SMPS control IC

Preliminary specification

TEA1507

Burst mode standby

Pin CTRL (pin 3) is also used to implement the burst mode

standby. In burst mode standby, the power supply enters

a special low dissipation state, where it typically consumes

less than 1 W of input power (Po < 100 mW), but is still

able to supply a microprocessor, for example. Figure 12

shows a flyback converter using the burst mode standby

function. The system enters burst mode standby when the

microprocessor closes switches S2 and S3 on the

secondary side. Switch S2 connects the high voltage

output secondary winding to the low voltage

microprocessor capacitor (CµC), bypassing Co.

When the voltage on CµC exceeds the Zener voltage, the

opto-coupler is activated, sending a large current signal to

CTRL. In response to this signal, the IC stops switching

and enters a ‘hiccup’ mode. This burst activation signal

should be present for longer than the ‘burst blank’ period

(typically 30 µs): the blanking time prevents false burst

triggering due to spikes. Figure 11 shows the burst-mode

standby signals. The hiccup mode during burst mode

standby operation does not differ from the hiccup mode in

safe-restart mode during a system fault condition (e.g.

OVP or output short circuit). The power is reduced

during soft-restart mode.

Burst mode standby operation continues until the

microcontroller opens switches S2 and S3. The system

then enters the start-up sequence and begins normal

switching behaviour.

Iburstmode = R-----VC----Tt--h-R---L- + Ith(on)

(For burst mode specification, see Figs 11 and 12.)

Soft start-up (pin Isense)

To prevent transformer rattle during hiccup, the

transformer peak current is slowly increased by the soft

start function. This can be achieved by inserting a resistor

and a capacitor between pin Isense (pin 5) and the sense

resistor. An internal current source charges the capacitor

to V = Iss × Rss, with a maximum of about 0.5 V.

The start level and the time constant of the increasing

primary current level can be adjusted externally by

changing the values of Rss and Css.

Iprimary(max) = V-----o--c---p----–R----(-s-I-e-s--n-s-s--×e-----R-----s--s---)

τ = Rss × Css

The charging current Iss will flow as long as the voltage on

pin Isense is below approximately 0.5 V. If the voltage on

pin Isense exceeds the 0.5 V, the soft start current source

will start limiting the current Iss. At the VCC(start) level, the Iss

current source is completely switched off (see Fig.9).

Since the soft start current Iss is subtracted from pin VCC

charging current, the Rss value will affect the VCC charging

current level by a maximum of 60 µA (typical value).

handbook, halfpage

0.5 V

ISS

start-up

pin 5

Isense

Vocp

RSS

CSS

Rsense

MGU237

Fig.9 Soft start-up.

Driver

The driver circuit to the gate of the power MOSFET has a

current sourcing capability of typically 125 mA and a

current sink capability of typical 540 mA. This permits fast

turn-on and turn-off of the power MOSFET for efficient

operation.

A low driver source current has been chosen to limit the

∆V/∆t at switch-on. This reduces Electro Magnetic

Interference (EMI) and also limits the current spikes

across Rsense.

2000 Dec 05

9

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