LT3485-3 データシートの表示（PDF） - Linear Technology

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LT3485-3

Photoflash Capacitor Chargers with Output Voltage Monitor and Integrated IGBT Drive

Linear Technology 

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OPERATIO

The LT3485-0/LT3485-1/LT3485-2/LT3485-3 are designed

to charge photoflash capacitors quickly and efficiently. The

operation of the part can be best understood by referring

to Figure 1. When the CHARGE pin is first driven high, a one

shot sets both SR latches in the correct state. The power

NPN device, Q1, turns on and current begins ramping up

in the primary of transformer T1. Comparator A1 monitors

the switch current and when the peak current reaches 2A

(LT3485-3), 1.4A (LT3485-0), 1A (LT3485-2) or 0.7A

(LT3485-1), Q1 is turned off. Since T1 is utilized as a flyback

transformer, the flyback pulse on the SW pin will cause the

output of A3 to be high. The voltage on the SW pin needs

to be at least 45mV higher than VBAT for this to happen.

During this phase, current is delivered to the photoflash

capacitor via the secondary and diode D1. As the second-

ary current decreases to zero, the SW pin voltage will begin

to collapse. When the SW pin voltage drops to 45mV above

VBAT or lower, the output of A3 (DCM Comparator) will go

low. This fires a one shot which turns Q1 back on. This cycle

will continue to deliver power to the output.

Output voltage detection is accomplished via R2, R1, Q2,

and comparator A2 (VOUT Comparator). Resistors R1 and

R2 are sized so that when the SW voltage is 31.5V above

VBAT, the output of A2 goes high which resets the master

latch. This disables Q1 and halts power delivery. NPN

transistor Q3 is turned on pulling the DONE pin low,

indicating that the part has finished charging. Power

delivery can only be restarted by toggling the CHARGE pin.

The CHARGE pin gives full control of the part to the user.

The charging can be halted at any time by bringing the

LT3485-0/LT3485-1/

LT3485-2/LT3485-3

CHARGE pin low. Only when the final output voltage is

reached will the DONE pin go low. Figure 2 shows these

various modes in action. When CHARGE is first brought

high, charging commences. When CHARGE is brought

low during charging, the part goes into shutdown and

VOUT no longer rises. When CHARGE is brought high

again, charging resumes. When the target VOUT voltage is

reached, the DONE pin goes low and charging stops.

Finally the CHARGE pin is brought low again so the part

enters shutdown and the DONE pin goes high.

Both VBAT and VIN have undervoltage lockout (UVLO).

When one of these pins goes below its UVLO voltage, the

DONE pin goes low. With an insufficient bypass capacitor

on VBAT or VIN, the ripple on the pin is likely to activate

UVLO and terminate the charge. The applications circuits

in the data sheet suggest values adequate for most

applications.

The LT3485 VMONT pin functions as an output to a

microcontroller to communicate the progress of the charge.

The VMONT pin starts to function at about 0.2V, which

corresponds to 64V with a turns ratio of 10.2. When the

VMONT pin is at 1V, the DONE pin goes low and the

charging terminates. The pin’s output is only valid when

the part is charging.

The LT3485 also integrates an IGBT drive. The IGBTPWR

pin supplies the power. The IGBT output goes high when

IGBTIN goes high and conversely goes low when IGBTIN

goes low. While IGBTIN is low, the IGBT drive draws no

quiescent current from IGBTPWR.

VOUT

100V/DIV

VDONE

5V/DIV

VCHARGE

5V/DIV

LT3485-2

VIN = 3.6V

COUT = 50µF

1s/DIV

3485 F02

Figure 2. Halting the Charging Cycle with the CHARGE Pin

34850123fb

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