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DV

(F)

(8)

DV

(F)

(9)

SYNCHRONIZING TO AN EXTERNAL SUPPLY

1

* 23 kW

(10)

65.27

(11)

SELECTING THE INDUCTOR VALUE

L +

(H)

(12)

TPS40060

TPS40061

SLUS543D–DECEMBER 2002–REVISED SEPTEMBER 2004

APPLICATION INFORMATION (continued)

The 10-V reference pin, BP10V needs to provide energy for the synchronous MOSFET gate drive via the BP10V

capacitor. Neglecting any efficiency penalty, the BP10V capacitance is described in Equation 9.

The TPS4006x can be synchronized to an external clock through the SYNC pin. The SW node rises on the

falling edge of the SYNC signal. The synchronization frequency should be in the range of 20% to 30% higher

than its programmed free-run frequency. The clock frequency at the SYNC pin replaces the master clock

generated by the oscillator circuit. Pulling the SYNC pin low programs the TPS4006x to freely run at the

Internally, the SYNC pin has a pull-down current between 5 µA and 10 µA. In order to synchronize the device to

an external clock signal, the SYNC pin has to be overdriven from the external clock circuit. Normal logic gates or

an external MOSFET with a pull-up resistor of 10 kΩ is adequate.

Internally there is a delay of between approximately 50 ns and 100 ns from the time the SYNC pin is pulled low

and the HDRV signal goes high to turn on the upper MOSFET. Additionally, there is some delay as the MOSFET

gate charges to turn on the upper MOSFET, typically between 20 ns and 50 ns.

The higher synchronization must be factored in when programming the PWM ramp generator circuit. If the PWM

ramp is interrupted by the SYNC pulse, a UVLO condition is declared and the PWM becomes disabled. Typically

value for RT that would cause the oscillator to run at the synchronizing frequency. Do not use this value of RT in

the design.

where:

•

The inductor value determines the magnitude of ripple current in the output capacitors as well as the load current

at which the converter enters discontinuous mode. Too large an inductance results in lower ripple current but is

physically larger for the same load current. Too small an inductance results in larger ripple currents and a greater

number of (or more expensive output capacitors for) the same output ripple voltage requirement. A good

compromise is to select the inductance value such that the converter doesn't enter discontinuous mode until the

load approximated somewhere between 10% and 30% of the rated output. The inductance value is described in

Equation 12.

where:

•

•∆I is the peak-to-peak inductor current

13