TFS7705H datasheet(12/42 Pages) POWERINT

12 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

The current limit selection for both FEEDBACK pin and ENABLE

pin takes place when the BYPASS pin first reaches 4.7 V. Once

the short detection period is complete, the BYPASS pin is

ramped on up to 5.7 V, and the FEEDBACK pin is allowed to

float to it’s nominal voltage of 3.5 V.

Standby Line Compensated Current Limit

to Flatten Output Overload Power

For many power supplies, the power output capability of the

power supply increases dramatically as the input voltage

increases. This means that most power supplies are able to

deliver much more power (up to 30-40% more power), into a

fault overload when operating at higher input voltage (versus

operating at lower input voltage). This can cause a problem

since many specifications require that the output overload

power capability of the device is more tightly managed.

In the case of the HiperTFS-2, the standby current limit is

adjusted as a function of line (input voltage), in such a ways as

to always provide substantially the same maximum overload

power capability. The input voltage is detected via the LINE-

SENSE pin current and the internal standby current limit of the

device is adjusted accordingly on a cycle-by-cycle basis. This

means that the HiperTFS-2 standby will only deliver approximately

5% more overload power at high-line as it did at low-line. This

feature provides a much safer design.

Standby Line Undervoltage Detection (UV)

The LINE-SENSE pin resistor is connected to V

IN and generates

a current signal proportional to V

IN. The LINE-SENSE pin

voltage is held by the device at 1.2 V. The LINE-SENSE pin

current signal is used to trigger under/overvoltage thresholds for

both the standby and main converters. Assuming a LINE-SENSE

pin resistor of 4 M

W, the standby will begin operating at

approximately 100 V (as defined by I

L(SB_UVON)). The standby will

shutdown if regulation is lost when input voltage is below 100 V.

However the standby will be forced to shutdown if this input

voltage drops below approximately 40 V (as defined by I

L(SB-UVOFF)).

Main and Standby Oscillator and Switching Frequency

The standby converter operates at a frequency of 132 kHz.

When in 66 kHz mode, main converter operates at exactly half

that frequency. The two converters both include a common

frequency jitter profile that varies the switching frequency ±4

kHz for the main (twice the jitter frequency range ±8 kHz for the

standby), during a 4 ms jitter period. The frequency jitter helps

reduce quasi-peak and average EMI emissions.

The HiperTFS-2 main switch frequency is selected at start-up

for 66 kHz or 132 kHz based on the value of the BYPASS pin

capacitor

It should be noted that when the HiperTFS-2 is running at 66 kHz

there is a collision avoidance scheme in which the main

converter is the master and the standby is the slave, which

avoids the main and standby switching at exactly simultaneous

moments. The most common condition would be close to 50%

duty cycle, if the main (master) is about to switch (turn-off), then

the standby (slave), waits for short instant (200 ns) before

starting it’s next cycle. The standby is used as the slave, since

the ON/OFF control of the HiperTFS-2 standby is less easily

disrupted by sudden delays in switching, versus the linear

control loop of the main converter. This collision avoidance is

not required when both standby and main run at 132 kHz.

Standby and Main Thermal Shutdown

The HiperTFS-2 provides a thermal shutdown function, (OTP)

that protects the HiperTFS-2. This hysteretic thermal shutdown

allows the device to automatically recover from any thermal fault

event. The thermal shutdown is triggered at a die-temperature

of approximately 118 °C and has a high hysteresis to ensure the

average device temperature is within safe levels. In a well

designed system the HiperTFS-2 thermal shutdown is not

triggered during any normal operation and is only present as a

safety feature to protect against abnormal or fault conditions.

BYPASS (BP) Pin Operation

The BYPASS (BP) pin is the supply pin for the entire HiperTFS-2

device. The BYPASS pin is internally connected to a high-voltage

current source via the STANDBY DRAIN power MOSFET. This

high-voltage source will charge the BYPASS pin to 4.7 V during

initial power-up. Once the BYPASS pin reaches 4.7 V, the

BYPASS pin will check the main and standby current limit

selection (FEEDBACK pin and ENABLE pin resistors respectively).

This selection takes a very short period, thereafter the BYPASS

pin continues being charged until it reaches 5.7 V. During this

change the value of the BYPASS pin capacitor is determined.

The value selects for main switching frequency of (1

mF = 66 kHz)

and 10

mF = 132 kHz) on completion of frequency selection, the

standby power supply is ready to begin operation. Like the

TinySwitch-4 the high-voltage current source will continue to

charge the BYPASS pin if it drops below 5.7 V. However in

most typical applications, a resistor (typically 7.5 k

W) is

connected from primary bias (12 V) to the BYPASS pin. This

resistor provides the operating current to the BYPASS pin,

preventing the need to draw power from the high-voltage

current source. Like the TinySwitch, the BYPASS pin contains a

Figure 12. Shows Output Overload Power for Compensated (TFS-2) Standby

and General Uncompensated Standby.

100

150

200

250

300

400

450

350

VIN DC (V)

150

140

130

120

100

110

Not Compensated

Compensated

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