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TEA1504

GreenChip™ SMPS control IC

Philips Electronics 

Philips Semiconductors

GreenChip™ SMPS control IC

Preliminary specification

TEA1504

Overvoltage protection

The OVP circuit senses the voltage at Vaux (pin 6). If the

output voltage exceeds the preset voltage limit, the OVP

circuit turns off the power MOSFET preventing the

re-supply of current to Caux. VVaux drops to the UVLO level

and the system enters the low dissipation safe-restart

mode described earlier. The system recovers from the

safe-restart mode only if the OVP condition is removed.

Overcurrent protection

Cycle-by-cycle OCP is provided by sensing the voltage

on Rsense. The voltage on Rsense relates to the amplitude

of the primary current, and is internally compared with a

reference voltage using a high speed comparator.

The comparator threshold voltage is specified as Vth(Isense)

in the Chapter “Characteristics”.

The maximum primary (protection) current is therefore:

Iprot = V-----tR-h---(-s-I-e-s--ne---sn---es---e--) [A] .

If the power MOSFET current exceeds the current limit, the

comparator changes state, turning off the power MOSFET.

The power MOSFET is typically turned off in 210 ns

(see td(Isense-DRIVER) in Chapter “Characteristics”).

Having Rsense off-chip allows the power supply designer

greater flexibility for programming the OCP threshold level.

It also reduces the risk of an overcurrent condition being

sensed incorrectly. When the power MOSFET turns on,

the discharge current from the demagnetization ∆V/∆t

limiting capacitor, flows through the power MOSFET

instead of through Rsense.

The Leading Edge Blanking circuit inhibits the operation of

the OCP comparator for a short period when the power

MOSFET turns on (see tblank(le) in Chapter

“Characteristics”). This ensures that the power 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 tblank(le) is not fixed and tracks the

oscillator frequency.

Overtemperature protection

Overtemperature protection is provided by an analog

temperature sensing circuit which turns off the power

MOSFET when the temperature exceeds typically 140 °C.

Figure 8 shows a flyback converter configured to use the

on/off mode. Switch S1 connects OOB (pin 14) to either a

voltage close to ground, or to a voltage typically greater

than 2.5 V. The OOB voltage is detected internally by

the IC. If VOOB is low, the IC enters the off-mode,

consuming a current of typically 350 µA (see Ioff(Vi) in

Chapter “Characteristics”). If VOOB is typically 2.5 V,

the IC enters the start-up sequence and begins normal

operation (see Vth(on/off) in Chapter “Characteristics”).

Figure 9 shows a ‘Mains Under Voltage Lock

Out’ (MUVLO) circuit using 3 resistors. Assuming that R3

is chosen to be a very high value, the IC starts operating

when: Vmains ≈ RR-----12-- × VOOB[V] ; where R1 >> R2.

This ensures that the power supply only starts working

above a Vmains of 80 V for example. The bleeder current

through R1 should be low (e.g. 30 µA at 300 V).

Burst mode standby

OOB (pin 14) 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 2 W of power. Figure 9 shows a flyback

converter using the burst mode standby function.

The system enters burst mode standby when the

microcontroller closes switches S2 and S3 on the

secondary side. Switch S2 connects the output secondary

winding to microcontroller capacitor (CµC) bypassing Co.

When the voltage on (CµC) exceeds the zener voltage, the

opto-coupler is activated sending a signal to OOB.

In response to this signal, the IC stops switching and

enters a ‘hiccup’ mode. Figure 7 shows the burst-mode

standby signals. The hiccup mode during burst mode

standby operation differs from the hiccup mode in

safe-restart mode during a system fault condition.

For safe-restart mode, the power has to be reduced.

Burst mode standby requires sufficient power to supply the

microcontroller. To prevent transformer rattle, the

transformer peak current is reduced by a factor of 3.

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.

On/off mode

The on/off mode allows an expensive mains switch to be

replaced by an in-expensive functional switch.

1999 Dec 07

9

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