TK654XX データシートの表示（PDF） - Toko America Inc

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TK654XX

STEP-DOWN CONVERTER WITH BATTERY MONITOR

Toko America Inc  

TK654xx

ADVANCED INFORMATION

THEORY OF OPERATION

Figure 1 illustrates a circuit diagram for a simple buck

(step-down) converter. Typically, the input voltage (VIN) is

greater than the output voltage (VOUT). By modulating the

switching action of switch SW1, the output voltage (VOUT)

can be regulated to a constant voltage that is relatively

independent of variations in the input supply (VIN) or the

current load on the VOUT node. The TK654xx contains all

the control circuitry, logic, and power switch (SW1) for

implementing a simple step-down or "buck" converter, as

shown in Figure 1.

In general, a switching converter utilizing the TK654xx

controller will be operating in one of three states:

1. "ON" STATE: During this state of operation SW1 will be

turned on. Current through the inductive element (L) will be

increasing at a rate proportional to the voltage difference

between VIN and VOUT. In this state, there is a direct

current path from the input supply to the output load

through the inductor L.

2. "OFF" STATE: During this state of operation SW1 will

be turned off. Current through the inductive element will be

greater than zero and flowing either through the external

schottky rectifier (D) or the synchronous rectifier internal to

the TK654xx. During the "off" state, current through the

inductive element (L) will be decreasing at a rate

approximately proportional to VOUT. In this state, the

current drawn from the input supply is essentially zero.

Current to the load is provided by stored energy in the

inductive element.

3. "NULL" STATE: During this state of operation SW1 will

be turned off. Current through the inductive element will be

approximately zero. The internal synchronous rectifier will

be turned off. All current demands of the load will be

provided by the output filter capacitor (C). In this state, the

current drawn from the input supply is essentially zero.

Since the inductive current is zero, no additional energy is

available from the inductor. If the current demands of the

load are very light, the current will be provided by the

stored charge in the output filter capacitor. If the voltage of

the filter capacitor drops below the regulation threshold, an

"on" state will be initiated and additional energy will be

transferred from the input supply to the output.

+

VIN

SW1

L

D

VOUT

C

FIGURE 1: SIMPLE "BUCK" CONVERTER

The control scheme for the TK654xx forces the converter

to step through the "on," "off," and "null" states in sequence.

Assume that the controller is initially in the "null" state and

VOUT is greater than the regulation threshold. As soon as

the output voltage drops below this threshold, the controller

will switch from "null" state to the "on" state. During the "on"

state, current through inductor (L) will be increasing.

Current will flow from the input supply to the output

capacitor through the inductive element. In this state,

energy is transferred directly from the input supply to the

output through the inductor. The maximum duration of the

"on" state is inversely proportional to the difference between

the input voltage (VIN) and the output voltage (VOUT).

The transition from the "on" state to the "off" state can be

initiated by one of two different means. As mentioned

above, the maximum duration of the "on" state is inversely

proportional to the difference between the input voltage

(VIN) and the output voltage (VOUT). If the duration of the

"on" cycle exceeds this maximum, the controller

immediately switches to the "off" state independent of

other factors. Understanding that when the converter is in

a dropout condition (VIN ≈ VOUT), the maximum “on” time

is infinite and the “on” state is constantly applied. By

limiting the duration of the "on" cycle, the peak inductor

current is also being limited. The second method for

initiating the "off" cycle is triggered when the duration of the

"on" cycle exceeds a minimum on-time duration and the

output voltage (VOUT) exceeds the regulation threshold.

Therefore, the actual duration of the "on" cycle will vary

between a minimum on-time (TON(MIN)) and a maximum

on-time (TON(MAX)) depending upon the load current. At

very light loads the on-time duration will be at a minimum;

at very heavy loads the on-time will be at a maximum. This

ability to vary the duration of the on cycle is a proprietary

control scheme which can produce a ten-fold reduction in

ripple when compared to competing devices.

The transition from the "off" state to the "null" state occurs

after sufficient time has been allowed for the inductor

current to return to zero. The actual duration of the "off"

Page 10

September 1999 TOKO, Inc.

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