7

LT1619

1619fa

OPERATIO

Inductor

The value of the inductor is usually selected so that the

peak-to-peak ripple current is less than 30% of the maxi-

mum inductor current. The inductor should be able to

handle the maximum inductor current at full load without

saturation. Powder iron cores are not suitable for high

frequency switch mode power supply applications be-

cause of their high core losses. Ferrite cores have very low

core losses and are the material of choice for high fre-

quency DC/DC converters.

Power MOSFET Driver

The LT1619 is capable of driving a low side N-channel

An external driver is recommended for MOSFETs with

greater than 80nC of total gate charge. The peak gate drive

gate of the power MOSFET to within 350mV of the upper

gate drive supply rail (DRV). It can also pull the gate of the

MOSFET to within 100mV of ground during turnoff. The

upper supply rail of the gate drive is brought out as a device

pin (DRV) for design flexibility. In a boost converter

design, the DRV pin can be tied to the converter output if

the minimum input voltage is insufficient to fully enhance

the power MOSFET. During start-up, the MOSFET is driven

forward voltage of the rectifying diode). As the output

voltage rises, the gate drive also increases until steady

state is reached. If the steady-state converter output

voltage exceeds the maximum allowable gate source

voltage and the input voltage is sufficient to enhance the

MOSFET, the DRV pin is tied to the input supply. For a

SEPIC converter, the DRV pin can be tied to the input or

diode OR’ed from the input and the output (Figure 4).

Figure 4. SEPIC Converter with Diode OR’ed Gate Drive Supply

or less than the number specified in Table 1 are accept-

able. The maximum duty cycle is essentially unaffected by

synchronization.

The device will go into shutdown mode if the S/S pin

voltage stays below the shutdown threshold of 0.45V for

more than 33

µs. This shutdown delay is reset whenever

the S/S pin voltage rises above the shutdown threshold.

Applying a logic low signal at the S/S pin causes the gate

drive output to go low. Although all circuits in the LT1619

are disabled, the pull-down circuit in the MOSFET buffer is

still biased on. It is capable of shunting any leakage or

transient current at the GATE pin to ground, eliminating

the need for an external bleed resistor. The LT1619 con-

sumes 15

µA in shutdown.

1.85V. Comparator A2 senses the input voltage and gen-

pulled low and the LT1619 stops switching. The supply

current drawn by the device falls to 140

µA.

Table 1. Maximum Allowable Rise Time of Synchronization

Pulse. Rise Time Can Be Slower if Clock Amplitude is Higher

SYNCHRONIZATION

MAXIMUM ALLOWABLE

AMPLITUDE (V)

RISE TIME (ns)

1.2

120

1.5

220

2.0

350

2.5

470

3.0

530

DRV

1619 F03

LT1619

GND

•

•

+

APPLICATIO S I FOR ATIO