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LTC3823

3823fd

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5V supply that powers the drivers and internal circuitry

within the LTC3823. The INTVCC pin can supply up to 50mA

RMS and must be bypassed to ground with a minimum

of 4.7μF low ESR tantalum capacitor or other low ESR

capacitor. Good bypassing is necessary to supply the high

transient currents required by the MOSFET gate drivers.

Applications using large MOSFETs with a high input voltage

and high frequency of operation may cause the LTC3823

to exceed its maximum junction temperature rating or

RMS current rating. Most of the supply current drives the

MOSFET gates. In continuous mode operation, this current

is IGATECHG=f(Qg(TOP)+Qg(BOT)).Thejunctiontemperature

can be estimated from the equations given in Note 2 of the

Electrical Characteristics. For example, the GN package is

limited to less than 23mA from a 30V supply:

TJ = 70°C + (23mA)(30V)(80°C/W) = 125°C

For applications where more current is needed than INTVCC

can supply, INTVCC can be driver by an external supply

with a voltage higher than 5.35V. However, the INTVCC pin

should not exceed its absolute maximum voltage of 7V.

External Gate Drive Buffers

The LTC3823 drivers are adequate for driving up to about

50nC into MOSFET switches with RMS currents of 50mA.

Applications with larger MOSFET switches or operating

at frequencies requiring greater RMS currents will beneﬁt

from using external gate drive buffers such as the LTC1693.

Alternately, the external buffer circuit shown in Figure 6

can be used.

Soft-Start and Tracking

The LTC3823 has the ability to either soft start by itself with

a capacitor or track the output of another supply. When

the device is conﬁgured to soft start by itself, a capacitor

should be connected to the TRACK/SS pin. The LTC3823

is put in a low quiescent current shutdown state (30μA)

if the RUN pin voltage is below 1.5V. The TRACK/SS

pin is actively pulled to ground in this shutdown state.

Once the RUN pin voltage is above 1.5V, the LTC3823 is

powered up. A soft-start current of 1.7μA then starts to

charge the soft-start capacitor CSS. Pin Z1/SSENABLE must

be grounded for soft-start operation. Note that soft-start

is achieved not by limiting the maximum output current

of the controller but by controlling the ramp rate of the

output voltage. Current foldback is disabled during this

soft-start phase. During the soft-start phase, the LTC3823

is ramping the reference voltage until it is 20% below the

voltage set by the VREFIN pin. The forced continuous mode

is also disabled and PGOOD signal is forced low during this

phase. The total soft-start time can be calculated as:

tSOFTSTART = 0.5V • CSS/1.7μA

When the device is conﬁgured to track another supply,

the feedback voltage of the other supply is duplicated by

a resistor divider and applied to the TRACK/SS pin. Pin

Z1/SSENABLE should be tied to INTVCC to turn off the soft-

start current in this mode. Therefore, the voltage ramp rate

on this pin is determined by the ramp rate of the other

supply output voltage.

Output Voltage Tracking

The LTC3823 allows the user to program how its output

ramps up and down by means of the TRACK/SS pin.

Through this pin, the output can be set up to either co-

incidentally or ratiometrically track with another supply’s

output, as shown in Figure 7. In the following discussions,

VOUT1 refers to the master LTC3823’s output and VOUT2

refers to the slave LTC3823’s output.

To implement the coincident tracking in Figure 7a, connect

an additional resistive divider to VOUT1 and connect its

midpoint to the TRACK/SS pin of the slave IC. The ratio of

this divider should be selected the same as that of the slave

IC’s feedback divider shown in Figure 8. In this tracking

APPLICATIONS INFORMATION

FMMT619

GATE

OF M1

BOOST

FMMT720

FMMT619

GATE

OF M2

3823 F06

INTVCC

PGND

FMMT720

10Ω

Figure 6. Optional External Gate Driver

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