ADP1876ACPZ-R7 datasheet(21/24 Pages) AD

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Data Sheet

ADP1876

Rev. A | Page 21 of 24

GATE

SINK

FALL

DRIVER

≅

where:

VDD is the input supply voltage to the driver and is between 2.75 V

and 5 V, depending on the input voltage.

VSP is the switching point where the MOSFET fully conducts;

this voltage can be estimated by inspecting the gate charge

graph given in the MOSFET data sheet.

RON_SOURCE is the on resistance of theADP1876 internal driver

(listed in Table 1), when charging the MOSFET.

RON_SINK is the on resistance of the ADP1876 internal driver

(listed in Table 1), when discharging the MOSFET.

RGATE is the on gate resistance of MOSFET listed in the MOSFET

data sheet. If an external gate resistor is added, add this external

resistance to RGATE.

The total power dissipation of the high-side MOSFET is the

sum of conduction and transition losses:

≅

The synchronous rectifier, or low-side MOSFET, carries the

inductor current when the high-side MOSFET is off. The low-

side MOSFET transition loss is small and can be neglected in

the calculation. For high input voltage and low output voltage,

the low-side MOSFET carries the current most of the time.

Therefore, to achieve high efficiency, it is critical to optimize

the low-side MOSFET for low on resistance. In cases where the

power loss exceeds the MOSFET rating or lower resistance is

required than is available in a single MOSFET, connect multiple

low-side MOSFETs in parallel. The equation for low-side

MOSFET conduction power loss is













−

≅

OUT

DSON

LOAD

CLS

)

(

There is an additional power loss during the time known as

dead time between the turn off of the high-side switch and the

turn on of the low-side switch when the body diode of the low-

side MOSFET conducts the output current.

The power loss in the body diode is given by

PBODYDIODE = VF × tD × fSW × IO

where:

VF is the forward voltage drop of the body diode, typically 0.7 V.

tD is the dead time in the ADP1876, typically 30 ns when driving

some medium-size MOSFETs with input capacitance, Ciss, of

approximately 3 nF. The dead time is not fixed. Its effective

value varies with gate drive resistance and Ciss thereby increasing

PBODYDIODE in high load current designs and low voltage designs.

Therefore, the power loss in the low-side MOSFET becomes

PLS = PCLS + PBODYDIODE

Note that MOSFET RDSON increases as temperature increases

with a typical temperature coefficient of 0.4%/oC. The MOSFET

junction temperature rise over the ambient temperature is

TJ = TA + θJA × PD

where:

θJA is the thermal resistance of the MOSFET package.

TA is the ambient temperature.

PD is the total power dissipated in the MOSFET.

LOOP COMPENSATION

As with most current mode step-down controllers, a trans-

conductance error amplifier is used to stabilize the external

voltage loop. Compensating the ADP1876 is fairly easy; an RC

compensator is needed between COMP and AGND. Figure 31

shows the configuration of the compensation components:

RCOMP, CCOMP, and CC2. Because CC2 is very small compared to

CCOMP, to simplify calculation, CC2 is ignored for the stability

compensation analysis.

Figure 31. Compensation Components

The open-loop gain transfer function at angular frequency, s, is

given by

)

(

)

(

)

(

FILTER

COMP

OUT

REF

(1)

where:

gm is the transconductance of the error amplifer, 500 µS

GCS is the tranconductance of the current sense amplifier.

ZCOMP is the impedance of the compensation network.

ZFILTER is the impedance of the output filter.

VREF = 0.6 V

GCS with units of A/V is given by

MIN

DSON

(2)

where:

ACS is the current sense gain of either 3 V/V, 6 V/V, 12 V/V, or

24 V/V set by the gain resistor between DLx and PGNDx.

RDSON_MIN is the the low-side MOSFET minimum on resistance.

If a sense resistor, RS, is added in series with the low-side FET,

then GCS becomes

)

(

MIN

DSON

ADP1876

FBx

CCOMP

0.6V

COMPx

AGND

RCOMP

CC2

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