BAT

OUT

BAT

L

OUT

9

9

± 9

L

,

¦

9

u

'

u

u

OUT

L

OUT

BAT

V

I

I

V

0.8



K

K

TRIP

BAT

REF

V

V

5

5

± 

 N

± 

V

0.5 V

§

·

§

·

u

: u

¨

¸

¨

¸

©

¹

©

¹

TPS61000, TPS61001, TPS61002, TPS61003

TPS61004, TPS61005, TPS61006, TPS61007

SLVS279D – MARCH 2000 – REVISED AUGUST 2015

www.ti.com

10.2.2.2 Programming the Low Battery Comparator Threshold Voltage

The current through the resistive divider should be about 100 times greater than the current into the LBI pin. The

typical current into the LBI pin is 0.01 µA. The voltage across R2 is equal to the reference voltage that is

generated on-chip, which has a value of 500 mV ±15 mV. The recommended value for R2 is therefore in the

range of 500 k

Ω. From that, the value of resistor R1, depending on the desired minimum battery voltage (VBAT),

can be calculated using the following equation:

(2)

For example, if the low-battery detection circuit should flag an error condition on the LBO output pin at a battery

voltage of 1 V, a resistor in the range of 500 k

Ω should be chosen for R1.

The output of the low battery comparator is a simple open-drain output that goes active low if the battery voltage

drops below the programmed threshold voltage on LBI. The output requires a pullup resistor with a

recommended value of 1M

Ω, and should only be pulled up to the VOUT. If not used, the LBO pin can be left

floating.

10.2.2.3 Inductor Selection

The output filter of inductive switching regulators is a low pass filter of second order. It consists of an inductor

and a capacitor, often referred to as storage inductor and output capacitor.

To select an inductor, keep the possible peak inductor current below the current limit threshold of the power

switch in your chosen configuration. For example, the current limit threshold of the TPS61006’s switch is 1100

mA at an output voltage of 3.3 V. The highest peak current through the inductor and the switch depends on the

output load, the input (VBAT), and the output voltage (VOUT). Estimation of the maximum average inductor current

can be done using the following equation:

(3)

For example, for an output current of 100 mA at 3.3 V, at least 515-mA current flows through the inductor at a

minimum input voltage of 0.8 V.

The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally it is

advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple reduces the

magnetic hysteresis losses in the inductor as well as output voltage ripple and EMI. But in the same way, the

regulation time at load change rises. In addition, a larger inductor increases the total system cost.

With those parameters it is possible to calculate the value for the inductor:

where

•

f is the switching frequency

•

ΔIL is the ripple current in the inductor, that is 20% x IL

(4)

In this example, the desired inductor has the value of 12 µH. With this calculated value and the calculated

currents, it is possible to chose a suitable inductor. Care has to be taken that load transients and losses in the

circuit can lead to higher currents as estimated in equation 3. Also, the losses in the inductor caused by magnetic

hysteresis losses and copper losses are a major parameter for total circuit efficiency.

The following inductors from different suppliers were tested. All work with the TPS6100x converter within their

specified parameters:

Table 3. Recommended Inductors

VENDOR

PART NUMBER

DO1608P Series

Coilcraft

DS1608P Series

DO3308 Series

16

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