LT3508 Просмотр технического описания (PDF) - Linear Technology

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LT3508

Dual Monolithic 1.4A Step-Down Switching Regulator

Linear Technology 

LT3508

APPLICATIONS INFORMATION

Setting the Output Voltage

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=

R2⎛⎝⎜

VOUT

0.8V

–

1⎞⎠⎟

R2 should be 20k or less to avoid bias current errors.

Reference designators refer to the Block Diagram.

Minimum Operating Voltage

The minimum operating voltage is determined either by

the LT3508’s undervoltage lockout or by its maximum duty

cycle. If VIN1 and VIN2 are tied together, the undervoltage

lockout is at 3.7V or below. If the two inputs are used

separately, then VIN1 has an undervoltage lockout of 3.7V

or below and VIN2 has an undervoltage lockout of 3V or

below. Because the internal supply runs off VIN1, chan-

nel 2 will not operate unless VIN1 > 3.7V. The duty cycle

is the fraction of time that the internal switch is on and is

determined by the input and output voltages:

DC = VOUT + VF

VIN – VSW + VF

Unlike many fixed frequency regulators, the LT3508 can

extend its duty cycle by turning on for multiple cycles. The

LT3508 will not switch off at the end of each clock cycle if

there is sufficient voltage across the boost capacitor (C3

in Figure 1). Eventually, the voltage on the boost capacitor

falls and requires refreshing. Circuitry detects this condi-

tion and forces the switch to turn off, allowing the inductor

current to charge up the boost capacitor. This places a

limitation on the maximum duty cycle as follows:

DCMAX

=

1+

1

1

βSW

where βSW is equal to the SW pin current divided by the

BOOST pin current as shown in the Typical Performance

Characteristics section. This leads to a minimum input

voltage of:

VIN(MIN)

=

VOUT + VF

DCMAX

–

VF

+

VSW

where VF is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.4V at maximum load).

Example: ISW = 1.5A and IBOOST = 50mA, VOUT = 3.3V,

βSW = 1.5A/50mA = 30, DCMAX = 1/(1+1/30) = 96%:

VIN(MIN)

=

3.3V + 0.4V

96%

–

0.4V

+

0.4V

=

3.8V

Maximum Operating Voltage

The maximum operating voltage is determined by the

Absolute Maximum Ratings of the VIN and BOOST pins,

and by the minimum duty cycle:

DCMIN = tON(MIN) • f

where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN)

is equal to 150ns) and f is the switching frequency.

Running at a lower switching frequency allows a lower

minimum duty cycle. The maximum input voltage before

pulse skipping occurs depends on the output voltage and

the minimum duty cycle:

VIN(PS)

=

VOUT + VF

DCMIN

– VF

+

VSW

Example: f = 790kHz, VOUT = 3.3V, DCMIN = 130ns • 790kHz

= 0.103:

VIN(PS)

=

3.3V + 0.4V

0.103

–

0.4V

+

0.4V

=

36V

The LT3508 will regulate the output current at input voltages

greater than VIN(PS). For example, an application with an

output voltage of 1.8V and switching frequency of 1.5MHz

has a VIN(PS) of 11.3V, as shown in Figure 2. Figure 3 shows

operation at 18V. Output ripple and peak inductor current

have significantly increased. Exceeding VIN(PS) is safe if

the output is in regulation, if the external components have

adequate ratings to handle the peak conditions and if the

peak inductor current does not exceed 3.2A. A saturating

inductor may further reduce performance. Do not exceed

VIN(PS) during start-up or overload conditions (for outputs

greater than 5V, use VOUT = 5V to calculate VIN(PS)). For

operation above 20V in pulse skipping mode, program

the switching frequency to 1.1MHz or less.

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