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LT1316

Micropower DC/DC Converter with Programmable Peak Current Limit

Linear Technology 

LT1316

APPLICATIONS INFORMATION

Discontinuous Mode Operation

A boost converter with a high VOUT:VIN ratio operates with

a high duty cycle in continuous mode. For duty cycles

exceeding the LT1316’s guaranteed minimum specifica-

tion of 0.73, the circuit will need to be designed for

discontinuous operation. Additionally, very low peak cur-

rent limiting below 50mA may necessitate operating in this

mode unless high inductance values are acceptable. When

operating in discontinuous mode, a different equation

governs available output power. For each switch cycle, the

inductor current ramps down to zero, completely releas-

ing the stored energy. Energy stored in the inductor at any

time is equal to 1/2 LI2. Because this energy is released

each cycle, the equation for maximum power out is:

POUT(MAX) = 1/2L(IPEAK2)f

)1

Where f =

IPEAK(L)

VIN – VSAT

+

tOFF

When designing for very low peak currents (< 50mA), the

inductor size needs to be large enough so that on-time is

a least 1µs. On-time can be calculated by the equation:

) On-Time =

IPEAK • L

(VIN – VSAT)

where VSAT = 0.2V.

Also, at these low current levels, current overshoot due to

power transistor turn-off delay will be a significant portion

of peak current. Increasing inductor size will keep this to

a minimum.

Design Example 1

Requirements: VIN = 2V, VOUT = 5V and ILOAD = 10mA.

1. Find duty cycle

) ) DC = VOUT – VIN + VD = 5 – 2 + 0.4 = 0.654

VOUT – VSAT + VD 5 – 0.2 + 0.4

Because duty cycle is less than the LT1316 minimum

specification (0.73), the circuit can be designed for

continuous operation.

2.

IPEAK

=

2(IOUT)

1 – DC

=

2(10mA)

1 – 0.654

=

58mA

3. Find L

) L =

VOUT – VIN + VD

0.4(IPEAK)

tOFF

) =

5 – 2 + 0.4

0.4(58mA)

2µs

= 293µH

4. Find RSET resistor

) Overshoot = VIN 300ns

L

) =

2

330µH

= 1.8mA

Find RSET from Figure 3 for 58mA – 1.8mA = 56.2mA

RSET ≈ 47k

Design Example 2

Requirements: VIN = 3.3V, VOUT = 28V and ILOAD = 5mA.

1. Find duty cycle:

) ) DC = VOUT – VIN + VD = 28 – 3.3 + 0.4 = 0.89

VOUT – VSAT + VD 28 – 0.2 + 0.4

Because duty cycle exceeds LT1316 minimum specifi-

cation of 73%, the circuit must be designed for discon-

tinuous operation.

2. Find POUT(MAX)

Multiply POUT by 1.4 to give a safe operating margin

POUT(MAX) = POUT(1.4) = (5mA)(28V)(1.4) = 0.196W

3. Set the on-time to the data sheet minimum of 3.4µs and

find L

L = (tON2)(VIN – VSAT)2

2POUT(MAX)(tON + tOFF)

=

(3.4µs2)(3.3 – 0.2)2

2(0.196W)(3.4µs + 2µs)

=

52µH

8

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