Functional Description

TSM006

7/13

Automatic burst mode

Burst mode is used during light load condition to

reduce the number of MOS switching, and thus

reducing overall power dissipation. Light load

condition is detected when COMP voltage is low.

When COMP voltage is lower than a threshold

is forced to off state, providing minimum duty

cycle and pulse skipping.

The burst status is available on the BS pin to put

other devices in standby mode when in light load

condition.

When COMP voltage (Vcomp) is higher than

Current is limited to (Vcomp-2*Vbe)/3 / Rshunt

(Rshunt is the shunt resistor used to measure the

primary current, Vbe is the forward voltage of a

diode, and 3 is the R/2R network attenuation).

Maximum current is 1V/Rshunt. When Vcomp

mode, PWM is stopped while Comp voltage is

energy is transferred to the secondary side,

output voltage is decreasing, and Vcomp (which is

an image of the error comparator) tends to

increase. As soon as Vcomp becomes just higher

2*Vbe)/3 / Rshunt.

OverCurrent detection and Hiccup mode

Overcurrent is detected when voltage at the CS

pin is greater than Vz2=1.2V. To avoid false

triggering, the overcurrent detection is delayed in

the same way than the normal pulse by pulse

current limitation.

When overcurrent is detected, the device enters

the

hiccup

mode.

Output

is

switched

off

immediately and the soft start capacitor is

discharged slowly. When the SS pin voltage goes

below 0.5V, normal soft start is started. If the

overcurrent is no more present, device operation

is resumed normally, otherwise, overcurrent is

detected again and the cycle is repeated until the

overcurrent situation disappears.

Duty cycle of the hiccup mode is set by the ratio of

SS pin discharge and charge currents: 10% typ.

With a typical capacitor Css=100nF, soft start

delay is about 40ms and hiccup off-time is 400ms.

Latched disable function

Disable mode is entered when the DIS pin voltage

is driven above 2.5V. Disable state is latched and

can only be exit by driving the Vin power supply

voltage under the UVLO level.

Thermal shutdown

The device operation is shut down when the

internal temperature exceed 130°C. Hysteresis

provides stable working and shutdown states.