LTC1262 データシートの表示（PDF） - Linear Technology

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LTC1262

12V, 30mA Flash Memory Programming Supply

Linear Technology 

LTC1262

APPLICATIONS INFORMATION

Operation

The LTC1262 uses a charge pump tripler to generate 12V

from a VCC of 5V. The charge pump operates when clocked

by a 300kHz oscillator. When the oscillator output is low,

C1 and C2 are connected between VCC and GND, charging

them to VCC. When the oscillator output goes high, C1 and

C2 are stacked in series with the bottom plate of C1 pulled

to VCC. The top plate of C2 is switched to charge COUT and

VOUT rises. VOUT is regulated to within 5% of 12V by an

oscillator pulse gating scheme. A resistor divider senses

VOUT. When the output of the divider (VDIV) is less than the

output of a bandgap (VBGAP) by the hysteresis voltage

(VHYST) of the comparator, oscillator pulses are applied to

the charge pump to raise VOUT. When VDIV is above VBGAP

by VHYST, the oscillator pulses are prevented from clock-

ing the charge pump. VOUT drops until VDIV is below VBGAP

by VHYST again. The gates of all internal switches are

driven between VOUT and GND. An internal diode ensures

that the LTC1262 will start up under load by charging COUT

to one diode drop below VCC.

To reduce supply current the LTC1262 may be put into

shutdown mode by floating the SHDN pin or taking it to

VCC. In this mode the bandgap, comparator, oscillator and

resistor divider are switched off to reduce supply current

to typically 0.5µA. At the same time an internal switch

shorts VOUT to VCC; VOUT takes 3.3ms to reach 5.1V (see

tOFF in Figure 1). When the SHDN pin is low, the LTC1262

exits shutdown and the charge pump operates to raise

VOUT to 12V. VOUT takes 500µs to reach the lower regula-

tion limit of 11.4V (see tON in Figure 1).

Choice of Capacitors

The LTC1262 is tested with the capacitors shown in

Figure 2. C1 and C2 are 0.22µF ceramic capacitors and

CIN and COUT are 4.7µF tantalum capacitors. Refer to

Table 1 if other choices are desired.

Table 1. Recommended Capacitor Types and Values

CAPACITOR

CERAMIC

TANTALUM

ALUMINUM

C1, C2

0.22µF to 1µF

Not

Not

Recommended Recommended

COUT

2µF (Min)

4.7µF (Min)

10µF (Min)

CIN

1µF (Min)

4.7µF (Min)

10µF (Min)

C1 and C2 should be ceramic capacitors with values in the

range of 0.22µF to 1µF. Higher values provide better load

regulation. Tantalum capacitors are not recommended as

the higher ESR of these capacitors degrades performance

when the load current is above 25mA with VCC = 4.75V.

CIN and COUT can be ceramic, tantalum or electrolytic

capacitors. The ESR of COUT introduces steps in the VOUT

waveform whenever the charge pump charges COUT. This

tends to increase VOUT ripple. Ceramic or tantalum capaci-

tors are recommended for COUT if minimum ripple is

desired. The LTC1262 does not require a 0.1µF capacitor

between VCC and VOUT for stability.

Maximum Load Current

The LTC1262 will source up to 50mA continuously without

any damage to itself. Do not short the VOUT pin to ground.

If the VOUT pin is shorted to ground, irreversible damage

to the device will result.

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