TK65127 データシートの表示（PDF） - Toko America Inc

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TK65127

STEP-UP VOLTAGE CONVERTER WITH VOLTAGE MONITOR

Toko America Inc  

TK651xx

SINGLE-CELL APPLICATION

The TK651xx is a boost converter control IC with the power

MOSFET switch built into the device. It operates from a

single battery cell and steps up the output voltage to a

regulated 2.7, 3.0 and 3.3 V. The device operates at a

fixed nominal clock frequency of 83 kHz.

In its simplest form, a boost power converter using the

TK651xx requires only three external components: an

inductor, a diode, and a capacitor.

The analysis is easier to follow when referencing the

simple boost circuit below.

filtering component values (consult the “Ripple and Noise

Considerations” section) can be determined if needed or

desired.

The TK651xx runs with a fixed oscillator frequency, and it

regulates by applying or skipping pulses to the internal

power switch. This regulation method is called Pulse Burst

Modulation (PBM).

ANALYSIS OF SWITCHING CYCLE

VIN

GND

SW

LOI

GND

VOUT

di/dt = VIN / L

IPEAK

di/dt = - (VOUT + Vf - VIN)/ L

VOUT

+

FIGURE 1: SIMPLE BOOST CONVERTER

t (on)

t (off)

t (deadtime)

THEORY OF OPERATION

The converter operates with one terminal of an inductor

connected to the DC input and the other terminal connected

to the switch pin of the IC. When the switch is turned on, the

inductor current ramps up. When the switch is turned off (or

“lets go” of the inductor), the voltage flies up as the inductor

seeks out a path for its current. A diode, also connected to

the switching node, provides a path of conduction for the

inductor current to the boost converter’s output capacitor.

The TK651xx monitors the voltage of the output capacitor

and has a 2.7, 3.0 and 3.3 V threshold at which the

converter switching becomes deactivated. So the output

capacitor charges up to 2.7, 3.0 and 3.3 V and regulates

there, provided that no more current is drawn from the

output than the inductor can provide. The primary task,

then, in designing a boost converter with the TK651xx

is to determine the inductor value (and its peak current

rating to prevent inductor core saturation problems)

which will provide the amount of current needed to

guarantee that the output voltage will be able to

maintain regulation up to a specified maximum load

current. Secondary necessary tasks also include choosing

the diode and the output capacitor. Then the snubber and

Above is the input or inductor current waveform over a

switching cycle.

From an oscillator standpoint, the switching cycle consists

of only an on-time and an off-time. But from an inductor

current standpoint, the switching cycle breaks down into

three important sections: on-time, off-time, and deadtime.

The on-time of the switch and the inductor current are

synonymous. During the on-time, the inductor current

increases. During the off-time, the inductor current

decreases as it flows into the output. When the inductor

current reaches zero, that marks the end of the inductor

current off-time. For the rest of the cycle, the inductor

current remains at zero. Since no energy is being either

stored or delivered, that remaining time is called “deadtime.”

This mode of the inductor current decaying to zero every

cycle is called “discontinuous mode.” In summary, energy

is stored in the inductor during on-time, delivered to the

output during off-time, and remains at zero during deadtime.

Page 10

January 1999 TOKO, Inc.

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