ADP2371ACPZ-1.2-R7 datasheet(26/32 Pages) AD

26 / 32 page

ADP2370/ADP2371

Data Sheet

Rev. A | Page 26 of 32

To minimize errors in the output voltage caused by the bias

current of the FB pin, maintain a value of R2 that is less than

250 kΩ. For example, when R2 and R3 each equal 250 kΩ, the

output voltage is 1.6 V. The output voltage error introduced by

the FB pin bias current is 2.5 mV, or 0.156%, assuming a typical

FB pin bias current of 10 nA at 25°C.

Note that in shutdown mode, the output is turned off and the

divider current is zero.

Select the output inductor and capacitor as described in the

Selecting the Inductor, Output Capacitor, and Input Capacitor

sections, as well as Table 6 for more information.

EFFICIENCY

Efficiency is defined as the ratio of output power to input power.

The high efficiency of the ADP2370/ADP2371 has two distinct

advantages. First, only a small amount of power is lost in the

dc-to-dc converter package, which in turn, reduces thermal

constraints. Second, high efficiency delivers the maximum

output power for the given input power, thereby extending

battery life in portable applications.

Power Switch Conduction Losses

Power switch dc conduction losses are caused by the flow of

output current through the P-channel power switch and the

N-channel synchronous rectifier, which have internal resis-

tances (RDS(ON)) associated with them. The amount of power

loss is approximated by

)

(

)

(

))

(

OUT

COND

−

where:

OUT

D =

The internal resistance of the power switches increases with tem-

perature and increases when the input voltage is less than 5.5 V.

Inductor Losses

Inductor conduction losses are caused by the flow of current

through the inductor, which has an internal resistance (DCR)

associated with it. Larger size inductors have smaller DCR,

which can decrease inductor conduction losses. Inductor core

losses relate to the magnetic permeability of the core material.

Because the ADP2370/ADP2371 are high switching frequency

dc-to-dc regulators, shielded ferrite core material is recommended

because of its low core losses and low EMI.

To estimate the total amount of power lost in the inductor, use

the following equation:

PL = DCR × IOUT2 + Core Losses

Switching Losses

Switching losses are associated with the current drawn by the

driver to turn-on and turn-off the power devices at the switching

frequency. Each time a power device gate is turned on and turned

off, the driver transfers a charge from the input supply to the

gate, and then from the gate to ground.

Estimate switching losses using the following equation:

PSW = (CGATE_P + CGATE_N) × VIN2 × fSW

where:

CGATE_P is the gate capacitance of the internal high-side switch.

CGATE_N is the gate capacitance of the internal low-side switch.

fSW is the switching frequency.

The typical value for gate capacitances, CGATE_P and CGATE_N,

is 150 pF.

Transition Losses

Transition losses occur because the P-channel switch cannot

turn on or turn off instantaneously. In the middle of an SW

node transition, the power switch provides all of the inductor

current. The source-to-drain voltage of the power switch is half

the input voltage, resulting in power loss. Transition losses

increase with both load current and input voltage and occur

twice for each switching cycle.

Use the following equation to estimate transition losses:

PTRAN = VIN/2 × IOUT × (tR + tF) × fSW

where:

tR is the rise time of the SW node.

tF is the fall time of the SW node.

The typical value for the rise and fall times, tR and tF, is 2 ns.

RECOMMENDED BUCK EXTERNAL COMPONENTS

The recommended external components for use with the

ADP2370/ADP2371 are listed in Table 6 (inductors) and Table 7

(capacitors).

FSEL

POWER GOOD

6.8µH

VOUT = 3.3V

VIN = 6V

CIN

10µF

COUT

10µF

AGND

(EXPOSED PAD)

VIN

SYNC

OFF

ADP2370/

ADP2371

PGND

Figure 82. Typical Application, 1.2 MHz, Fixed Output

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