REV. 0

ADP3170

–13–

In continuous inductor-current mode, the source current of the

current. To prevent large voltage transients, a low ESR input

capacitor sized for the maximum rms current must be used.

The maximum rms capacitor current is given by:

II

D

D

IA

A

C RMS

O

HSF

HSF

C RMS

()

()

–

.–.

.

=

=×

=

2

2

23

0 4

0 4

11 3

(27)

For a ZA-type capacitor with 1000

µF capacitance and 6.3 V

voltage rating, the ESR is 24 m

Ω and the maximum allowable

ripple current at 100 kHz is 2 A. At 105

°C, at least six such

capacitors must be connected in parallel to handle the calcu-

lated ripple current. At 50

°C ambient, however, a higher

ripple current can be tolerated, so five capacitors in parallel

are adequate.

The ripple voltage across the five paralleled capacitors is:

VI

ESR

n

D

nC

f

VA

m

mF

kHz

mV

C RIPPLE

O

C

C

HSF MAX

CIN

MIN

C RIPPLE

()

()

()

.

=×

+

××









=×

×

××









=

23

24

5

04

5 1

183

120

Ω

(28)

To further reduce the effect of the ripple voltage on the system

supply voltage bus and to reduce the input-current di/dt to

below the recommended maximum of 0.1 A/ms, an additional

small inductor (L > 1

µH @ 10 A) should be inserted between

the converter and the supply bus.

Linear Regulators

The linear regulator provides a low cost, convenient and versa-

tile solution for generating a 1.8 V supply rail. The maximum

output load current is determined by the size and thermal

impedance of the external N-channel power MOSFET that is

placed in series with the supply and controlled by the ADP3170.

The output voltage is sensed at the LRFB pin and compared to

an internal reference voltage in a negative feedback loop which

keeps the output voltage in regulation. If the load is reduced or

increased, the MOSFET drive will also be reduced or increased

by the ADP3170 to provide a well regulated output voltage.

Output voltages higher than the fixed internal reference voltage

can be programmed by adding an external resistor divider.

Efficiency of the Linear Regulators

The efficiency and corresponding power dissipation of each

of the linear regulators are not determined by the ADP3170.

Rather, these are a function of input and output voltage and

load current. Efficiency is approximated by the formula:

η=

×

100%

V

V

OUT

IN

(29)

The corresponding power dissipation in the MOSFET, together

with any resistance added in series from input to output is given by:

PV

V

I

LDO

IN

OUT

OUT

=

–

(30)

Minimum power dissipation and maximum efficiency are

accomplished by choosing the lowest available input voltage

that exceeds the desired output voltage. However, if the chosen

input source is itself generated by a linear regulator, its power

dissipation will be increased in proportion to the additional

current it must now provide.

3.3V

ADP3170

1.8V

LRDRV

LRFB

1k

250m

1 F

1.8V, 2.2A

68pF

100 F

10k

Figure 4. Adding Overcurrent Protection to the Linear

Regulator

Implementing Current Limit for the Linear Regulators

The circuit of Figure 4 gives an example of a current limit pro-

tection circuit that can be used in conjunction with the linear

regulator. The output voltage is internally set by the LRFB pin.

The value of the current sense resistor may be calculated as

follows:

R

mV

I

mV

A

m

S

O MAX

≅==

540

540

22

250

()

.

Ω

(31)

The power rating of the current sense resistor must be at least:

PR

I

W

D R

S

O MAX

S

()

(

)

.

=×

=

2

12

(32)

The maximum linear regulator MOSFET junction temperature with

a shorted output is:

TT

V

I

TC

C W

V

A

C

MAX

A

C

IN

O MAX

MAX

JJ

J

()

()

()

./

.

.

=+

×

×

()

=+

×

×

θ

50

14

33

22

60

oo

o

(33)

which is within the maximum allowed by the MOSFET’s data

sheet specification. The maximum MOSFET junction tempera-

ture at nominal output is:

This example assumes an infinite heat sink. The practical limita-

tion will be based on the actual heat sink used.

TC

V

V

I

TC

C W

V

V

A

C

NOM

C

IN

OUT

O NOM

NOM

JJ

J

()

()

()

–

./

.

– .

=+

×

()

=+

×

50

50

14

33

18

2

54

o

oo

o

θ

(34)