LTC3442 datasheet(8/16 Pages) LINER

8 / 16 page

LTC3532

3532fc

Buck Region (VIN > VOUT)

Switch D is always on and switch C is always off dur-

ing this mode. When the internal control voltage, VCI, is

above voltage V1, output A begins to switch. During the

off-time of switch A, synchronous switch B turns on for

the remainder of the time. Switches A and B will alternate

like a typical synchronous buck regulator. As the control

voltage increases, the duty cycle of switch A increases

until the maximum duty cycle of the converter in buck

mode reaches DMAX_BUCK, given by:

DMAX_BUCK = 100 – D4SW %

where D4SW = duty cycle % of the 4-switch range.

D4SW = (150ns • f) • 100 %

where f = operating frequency, Hz.

Beyond this point the “4-switch,” or buck/boost region

is reached.

Buck/Boost or 4-Switch (VIN ~ VOUT)

When the internal control voltage, VCI, is above voltage

V2, switch pair AD remain on for duty cycle DMAX_BUCK,

and the switch pair AC begins to phase in. As switch pair

AC phases in, switch pair BD phases out accordingly.

When the VCI voltage reaches the edge of the buck/boost

range, at voltage V3, the AC switch pair completely phase

out the BD pair, and the boost phase begins at duty cycle

D4SW. The input voltage, VIN, where the 4-switch region

begins is given by:

OPERATIO

88%

DMAX

BOOST

DMIN

BOOST

DMAX

BUCK

DUTY

CYCLE

V4 (≈2.05V)

V3 (≈1.65V)

BOOST REGION

BUCK REGION

BUCK/BOOST REGION

V2 (≈1.55V)

V1 (≈0.9V)

3532 F02

A ON, B OFF

PWM CD SWITCHES

D ON, C OFF

PWM AB SWITCHES

FOUR SWITCH PWM

INTERNAL

CONTROL

VOLTAGE, VCI

Figure 2. Switch Control vs Internal Control Voltage, VCI

VIN =

VOUT

1– (150ns • f)

The point at which the 4-switch region ends is given by:

VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V

Boost Region (VIN < VOUT)

Switch A is always on and switch B is always off dur-

ing this mode. When the internal control voltage, VCI, is

above voltage V3, switch pair CD will alternately switch

to provide a boosted output voltage. This operation is like

a synchronous boost regulator. The maximum duty cycle

of the converter is limited to 88% typical and is reached

when VCI is above V4.

Burst Mode OPERATION

Burst Mode operation occurs when the IC delivers energy

to the output until it is regulated and then goes into a sleep

mode where the outputs are off and the IC is consuming

only 35μA of quiescent current from VIN. In this mode the

output ripple has a variable frequency component that

depends upon load current, and will typically be about

2% peak-to-peak. Burst Mode operation ripple can be

reduced slightly by using more output capacitance (47μF

or greater). Another method of reducing Burst Mode

operation ripple is to place a small feedforward capacitor

across the upper resistor in the VOUT feedback divider

network (as in Type III compensation). During the period

where the device is delivering energy to the output, the

peak switch current will be equal to 250mA typical and

the inductor current will terminate at zero current for each

cycle. In this mode the typical maximum average output

current is given by:

IOUT(MAX)BURST ≈

0.2 • VIN

VOUT + VIN

LTC3442 Datasheet(PDF) 8 Page - Linear Technology

LTC3442 Datasheet(HTML) 8 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL