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KB3511

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

OPERATION

voltage until the average inductor current matches the new

load current.

The main control loop is shut down by pulling the RUN pin

to ground.

Low Current Operation

Two modes are available to control the operation of the

KB3511 at low currents. Both modes automatically switch

from continuous operation to to the selected mode when

the load current is low.

To optimize efficiency, the Trickle Mode operation can be

selected. When the load is relatively light, the KB3511

automatically switches into Trickle Mode operation in which

the PMOS switch operates intermittently based on load

demand with a fixed peak inductor current. By running

cycles periodically, the switching losses which are domi-

nated by the gate charge losses of the power MOSFETs are

minimized. The main control loop is interrupted when the

output voltage reaches the desired regulated value. A

0.28V, shutting off the switch and reducing the power. The

output capacitor and the inductor supply the power to the

the main control loop which starts another cycle.

For lower ripple noise at low currents, the pulse skipping

mode can be used. In this mode, the KB3511 continues

to switch at a constant frequency down to very low

currents, where it will begin skipping pulses.

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases to 100% which is

the dropout condition. In dropout, the PMOS switch is

turned on continuously with the output voltage being

equal to the input voltage minus the voltage drops across

the internal p-channel MOSFET and the inductor.

the P-channel switch increases with decreasing input

supply voltage (See Typical Performance Characteristics).

Therefore, the user should calculate the power dissipation

when the KB3511 is used at 100% duty cycle with low

input voltage (See Thermal Considerations in the Applica-

tions Information Section).

Low Supply Operation

The KB3511 incorporates an Under-Voltage Lockout

circuit which shuts down the part when the input voltage

drops below about 1.65V to prevent unstable operation.

A general KB3511 application circuit is shown in

Figure 2. External component selection is driven by the

load requirement, and begins with the selection of the

be selected.

Inductor Selection

Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current

∆=

⎛

⎝⎜

⎞

⎠⎟

I

V

fL

V

V

L

OUT

O

OUT

IN

1

Accepting larger values of

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

The largest ripple current

input voltage. To guarantee that the ripple current stays

below a specified maximum, the inductor value should be

chosen according to the following equation:

L

V

fI

V

V

OUT

OL

OUT

IN MAX

=

∆

⎛

⎝

⎜

⎞

⎠

⎟

()

1

The inductor value will also have an effect on Trickle Mode

operation. The transition from low current operation be-

gins when the peak inductor current falls below a level set

by the Trickle clamp. Lower inductor values result in higher

ripple current which causes this to occur at lower load

APPLICATIONS INFORMATION