ADP1829ACPZ-R7 datasheet(24/32 Pages) AD

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ADP1829

Rev. 0 | Page 24 of 32

Setting the Channel 2 Undervoltage Threshold for

Ratiometric Tracking

THERMAL CONSIDERATIONS

The current required to drive the external MOSFETs comprises

the vast majority of the power dissipation of the ADP1829. The

on-chip LDO regulates down to 5 V, and this 5 V supplies the

drivers. The full gate drive current passes through the LDO and

is then dissipated in the gate drivers. The power dissipated on

the gate drivers on the ADP1829 is

If FB2 is regulated to a voltage lower than 0.6 V by configuring

TRK2 for ratiometric tracking, the Channel 2 undervoltage

threshold can be set appropriately by splitting the top resistor in

the voltage divider, as shown in Figure 33. RBOT is the same as

calculated for the compensation in Equation 52, and

TOP

(53)

)

(

(57)

550mV

750mV

POK2

UV2

FB2

CHANNEL 2

OUTPUT

VOLTAGE

RBOT

TO ERROR

AMPLIFIER

where:

VIN is the voltage applied to IN.

fSW is the switching frequency.

Q numbers are the total gate charge specifications from the

selected MOSFET data sheets.

The power dissipation heats the ADP1829. As the switching

frequency, the input voltage, and the MOSFET size increase, the

power dissipation on the ADP1829 increases. Care must be taken

not to exceed the maximum junction temperature. To calculate

the junction temperature from the ambient temperature and

power dissipation,

(58)

Figure 33. Setting the Channel 2 Undervoltage Threshold

The current in all the resistors is the same:

OUT

BOT

−

(54)

The thermal resistance, θJA, of the package is typically 40°C/W

depending on board layout, and the maximum specified

junction temperature is 125°C, which means that at maximum

ambient of 85°C without airflow, the maximum dissipation

allowed is about 1 W.

where:

VUV2 is 600 mV.

VFB2 is the feedback voltage value set during the ratiometric

tracking calculations.

VOUT2 is the Channel 2 output voltage.

A thermal shutdown protection circuit on the ADP1829 shuts

off the LDO and the controllers if the die temperature exceeds

approximately 145°C, but this is a gross fault protection only

and should not be relied upon for system reliability.

Solving for RA and RB,

(

)

OUTA

BOT

−

(55)

(

)

BOT

−

(56)

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