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UT2306-3.3V

MICROPOWER DC/DC CONVERTERS WITH LOW-BATTERY DETECTOR ACTIVE IN SHUTDOWN

Unisonic Technologies 

UTC UT2306/UT2306-3.3V/UT2306-5.0V

LINEAR INTEGRATED CIRCUIT

1

2 LBO

VIN

3

4 SW

8

LBI

+

-

A3

1.17V

1.5V

UNDERVOLTA

GELOCKOUT

A2

36mV

+

R2

-

1K

R1

7.2Ω

R3

590K

R4

-

A1

+

1.24V

VREF

OFF

ENABLE

TIMERS

6μs ON

1.5μs OFF

BIAS

-1V

Q3

1K

Q2

DRIVER

×1

Q1

×200

R4=355K (UT2306-3.3V)

R4=195K (UT2306-5.0V)

Figure 2. UT2306-3.3/UT2306-5.0 Block Diagram

SHUTDOWN

7 SHDN

ILIM

6

GND

5

OPERATION

The UT2306 operation can best be understood by examining the block diagram in Figure 1.Comparator A1monitors

the output voltage via resistor divider string R3/R4 at the FB pin. When VFB is higher than the 1.24V reference,A2

and the timers are turned off. Only the reference, A1 and A3 consume current, typically 120µA.As VFB drops below

1.24V plus A1’s hysteresis (about 6mV),A1 enables the rest of the circuit. Power switch Q1 is then cycled on for 6µs,

or until current comparator A2 turns off the ON timer, Whichever comes first. Off-time is fixed at approximately 1.5µs.

Q1’s switching cause current to alternately build up in inductor L1 and discharge into output capacitor C2 via D1,

increasing the output voltage .As VFB increases enough to overcome C1’s hysteresis, switching action ceases. C2 is

left to supply current to the load until VOUT decreases enough to force A1’s output high, and the entire cycle repeats.

If switch current reaches 1A,causing A2 to trip, switch ON time is reduced. This allows continuous mode operation

during bursts.A2 monitors the voltage across 7.2Ωresistor R1,which is directly related to the switch current.Q2’s

collector current is set by the emitter-area ratio to 0.5% of Q1’s collector current. R1’s voltage drop exceeds

36mV,corresponding to 1A switch current,A2’s output goes high ,truncating the ON time part of the switch cycle. The

1A peak current can be reduced by tying a resistor between the ILIM pin and ground, causing a voltage drop to

appear across R2.The drop offsets some of the 36mV reference voltage, lowering peak current. A 22K resistor limits

current to approximately 550mA.A capacitor connected between ILIM and ground provides soft start. Shutdown is

accomplished by grounding the SHDN pin.

The low-battery detector A3 has its own 1.17V reference and is always on. The open collector output device can sink

up to 500µA.Approximately 35mV of hysteresis is built into A3 to reduce ”buzzing” as the battery voltage reaches the

trip level.

INDUCTOR SELECTION

Inductors used with the UT2306 must be capable of handling the worst-case peak switch current of 1.2A

without saturating. Open flux rod or drum core units may be biased into saturation by 20% with only a small

reduction in efficiency. For the majority of 2-cell or 3-cell input UT2306 applications, a 22µH or 20µH inductor such

as the Sumida CD54-220 (drum) or Coiltronics CTX20-1 (toroid) will suffice. If switch current is reduced using the

ILIM pin, smaller inductors such as the Sumida CD43 series or Coilcraft DO1608 series can be used. Minimizing

UTC UNISONIC TECHNOLOGIES CO., LTD. 6

QW-R103-029,A

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