LT1616ES6 데이터 시트보기 (PDF) - Linear Technology

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LT1616ES6

600mA, 1.4MHz Step-Down Switching Regulator in SOT-23

Linear Technology 

LT1616

APPLICATIO S I FOR ATIO

The LT1616 efficiently converts power from an input volt-

age source to a lower output voltage using an inductor for

energy storage. The LT1616 uses its internal power switch

and an external catch diode (D1 of the application circuit

on the first page of this data sheet) to produce a pulse-

width modulated square wave. Inductor L1 and output

capacitor C2 filter this square wave to produce a DC output

voltage. An error amplifier regulates the output by com-

paring the output (divided by the feedback resistor string

R1 and R2) to an internal reference. The LT1616 uses

current mode control; instead of directly modulating the

pulse width, the error amplifier controls the peak current

in the switch and inductor. Current mode control has sev-

eral advantages, including simplified loop compensation

and cycle-by-cycle current limiting.

Figure 1 shows several waveforms of the application cir-

cuit on the front page of this data sheet. The circuit is

converting a 12V input to 3.3V at 300mA. The first trace is

the voltage at the SW pin. When the internal switch is on,

the SW pin voltage is near the 12V input. This applies a

voltage across inductor L1, and the current in the switch

(second trace) and the inductor (third trace) increases.

When the switch turns off, the switch current immediately

drops to zero and the inductor current flows through the

catch diode D1, which clamps the switch node 0.4V below

ground. The voltage across the inductor in this state has

the opposite sense and is equal to the output voltage plus

the catch diode drop, so the inductor current begins to

decrease. The fourth trace shows the output voltage ripple.

At light loads, the inductor current may reach zero on each

pulse. The diode will turn off, and the switch voltage will

ring, as shown in Figure 2. This is discontinuous mode op-

eration, and is normal behavior for the switching regula-

tor. The LT1616 will also skip pulses when the load is light.

VSW

5V/DIV

IL1

0.2A/DIV

VSW

5V/DIV

ISW

0.2A/DIV

IL1

0.2A/DIV

VOUT

5mV/DIV

200ns/DIV

1616 F01a

200ns/DIV

1616 F01b

Figure 1. Operating Waveforms of the LT1616

Converting 12V to 3.3V at 300mA

6

VIN = 12V

VOUT = 5V

IOUT = 18mA

500ns/DIV

1616 F02

Figure 2. Discontinuous Mode Operation

If the output is shorted to ground, the output voltage will

collapse and there will be very little voltage to reset the

current in the inductor. The LT1616 can sense this condi-

tion at its FB pin. In order to control the current, the LT1616

reduces its operating frequency, allowing more time for

the catch diode to reset the inductor current.

The input and output voltages determine the duty cycle of

the switch. The inductor value combined with these volt-

ages determines the ripple current in the inductor. Along

with the switch current limit, the inductor ripple current

determines the maximum load current that the circuit can

supply. At minimum, the input and output capacitors are

required for stable operation. Specific values are chosen

based on allowable ripple and desired transient perfor-

mance. The rest of the applications information is mainly

concerned with choosing these and the other components

in an LT1616 application.

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