TFS7701-7708 datasheet(9/42 Pages) POWERINT

9 / 42 page

Rev. B 04/15

TFS7701-7708

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regulation before the main applies a load to the bulk-capacitor.

Also during this pre-charge period the high-side driver is

charged via the bootstrap diode (or self bias) from the low-side

auxiliary voltage, and is charged when the main low-side

MOSFET turns on, while the main high-side MOSFET is held off.

By the end of the pre-charge period, the PFC-boost voltage

should be at or above the nominal boost voltage. The

HiperTFS-2 begins switching, going through the soft-start

period (t

SS). During the soft-start period the maximum duty

cycle starts at 30% and is ramped during a 12 ms period to the

maximum. The ramped duty cycle controls the rise slew rate of

the output during start-up, allowing well controlled start-up and

also facilitates a smooth transition when the control loop takes

over regulation towards the end of soft-start. This allows the

main to start-up within the required period for the application

(typically <20 ms for PC main applications), when there is a

substantial capacitive load on the output.

Main Converter Control FEEDBACK (FB) Pin Operation

The FEEDBACK pin is the input for control loop feedback from

the main control loop. During normal operation the FEEDBACK

pin is used to provide duty cycle control for the main converter.

The system output voltage is detected and converted into a

feedback current. The main converter duty cycle will reduce as

more current is sourced from the FEEDBACK pin, reaching zero

duty cycle at approximately 2.1 mA. The nominal voltage of the

FEEDBACK pin is maintained at approximately 3.5 V. An

internal pole on the FEEDBACK pin is set to approximately

12 kHz, in order to facilitate optimal control loop response.

The maximum duty cycle of the main converter is defined by the

LINE-SENSE pin and RESET pin behavior and is a dynamically

calculated value according to cycle-by-cycle conditions on the

LINE-SENSE pin and RESET pin.

Main High-Side Driver

The high-side driver is a device that is electrically floating at the

potential of the HIGH-SIDE MOSFET SOURCE (HS) pin. This

device provides gate-drive for the high-side main MOSFET. The

low-side main and high-side main MOSFET’s switch simultaneously.

The high-side driver has a HIGH-SIDE OPERATING VOLTAGE

supply pin. External circuitry or an internal source, provides

operating current and provides HIGH-SIDE OPERATING

VOLTAGE pin. The high-side operating voltage has an internal

12 V shunt-regulator. The device consumes approximately 2.3 mA

when driving the high-side MOSFET.

The HIGH-SIDE OPERATING VOLTAGE pin has an undervoltage

lock-out threshold, to prevent gate-drive when the supply voltage

drops below a safe threshold. At power-up the high-side driver

remains in the off-state, until the HIGH-SIDE OPERATING

VOLTAGE pin is charged above 10.5 V, at which point the

high-side driver becomes active. The high-side driver is initially

charged via either a bootstrap diode connected to the HIGH-

SIDE OPERATING VOLTAGE pin from the low-side standby

auxiliary supply (approximately 12 V) or from an internal high-

voltage current source. During start-up the high-side MOSFET

remains off, but the low-side MOSFET is turned on for a period

of 60 ms to allow pre-charge of the high-side operating voltage

to 12 V. After this period, the high-side operating voltage is

supplied by a bootstrap diode or internal current source.

Once the high-side driver is operating it receives level-shifted

drive commands from the low-side device. These drive

commands cause both turn-on and turn-off drive of the

high-side main MOSFET simultaneously with that of the

low-side main MOSFET.

The high-side driver also contains a thermal shutdown on-chip,

but this is set to a temperature above the thermal shutdown

temperature of the low-side device. Thus the low-side will

always shutdown first.

Main Converter Maximum Duty Cycle

The LINE-SENSE pin resistor converts the input voltage into an

LINE-SENSE pin current signal. The RESET pin resistor

converts the reset voltage into an RESET pin current signal.

The LINE-SENSE pin and RESET pin currents allow the

HiperTFS-2 to determine a maximum duty cycle envelope on a

cycle-by-cycle basis. This feature ensures sufficient time for

transformer reset on a cycle-by-cycle basis and also protects

against single-cycle transformer saturation and at high-input

voltage by limiting the maximum duty cycle to prevent the

transformer from reaching an unsafe flux density during the

on-time period. Both of these features allow the optimal

performance to be obtained from the main transformer. The

duty cycle limit is trimmed during production.

The LINE-SENSE pin and RESET pin are sampled just before

the turn-on of the next main cycle. This is done to sample at a

point when there is minimal noise in the system. Due to the low

current signal input to the LINE-SENSE pin and RESET pin,

care should be taken to prevent noise injection on these pins

(see Applications section layout guidelines for details).

Main On-Chip Current Limit with External Selection

During start-up, the FEEDBACK pin and ENABLE pin are both

used to select internal current limits for the main and standby

converters respectively. The detection period occurs at the

initial start-up of the device, and before the main or standby

MOSFETs start switching. This is done to minimize noise

interference.

Figure 7.

PWM Duty Cycle vs. Control Current.

61%

70%

1.1 mA

2.1 mA

Duty (D)

FEEDBACK Pin

Sink Current IFB

L = 90 µA

R = 170 µA

Typical IL and IR

currents at VMIN

PI-6913-053113

Limited by L and R

pin duty limit

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