9

FN6353.0

July 2, 2007

Operation

The boost converter is a current mode PWM converter

operating at either a 650kHz or 1.2MHz. It can operate in

both discontinuous conduction mode (DCM) at light load and

continuous mode (CCM). In continuous current mode,

current flows continuously in the inductor during the entire

switching cycle in steady state operation. The voltage

conversion ratio in continuous current mode is given by

Equation 1:

Where D is the duty cycle of the switching MOSFET.

Figure 12 shows the block diagram of the boost regulator. It

uses a summing amplifier architecture consisting of gm

stages for voltage feedback, current feedback and slope

compensation. A comparator looks at the peak inductor

current cycle by cycle and terminates the PWM cycle if the

current limit is reached.

An external resistor divider is required to divide the output

voltage down to the nominal reference voltage. Current

drawn by the resistor network should be limited to maintain

the overall converter efficiency. The maximum value of the

resistor network is limited by the feedback input bias current

and the potential for noise being coupled into the feedback

pin. A resistor network in the order of 60k

Ω is recommended.

The boost converter output voltage is determined by

Equation 2:

This restricts the maximum output current (average) based

on Equation 3:

Where

ΔI

by Equation 4:

Table 2 gives typical values (margins are considered 10%,

Capacitor

An input capacitor is used to suppress the voltage ripple

injected into the boost converter. The ceramic capacitor with

capacitance larger than 10µF is recommended. The voltage

rating of input capacitor should be larger than the maximum

input voltage. Some capacitors are recommended in Table 1

for input capacitor.

V

Boost

V

IN

-------------------

1

1D

–

-------------

=

(EQ. 1)

V

Boost

R

1

R

2

+

R

2

---------------------

V

FB

×

=

(EQ. 2)

TABLE 1. BOOST CONVERTER INPUT CAPACITOR

RECOMMENDATION

CAPACITOR

SIZE

MFG

PART NUMBER

10µF/16V

1206

TDK

C3216X7R1C106M

10µF/10V

0805

Murata

GRM21BR61A106K

22µF/10V

1210

Murata

GRB32ER61A226K

I

OMAX

I

LMT

ΔI

L

2

--------

–

⎝⎠

V

O

---------

×

=

(EQ. 3)

ΔI

L

V

IN

L

---------

D

f

s

----

×

=

(EQ. 4)

TABLE 2. MAXIMUM OUTPUT CURRENT CALCULATION

L (µH)

3

9

10

0.65

636

3

12

10

0.65

419

3

15

10

0.65

289

5

9

10

0.65

1060

5

12

10

0.65

699

5

15

10

0.65

482

5

18

10

0.65

338

3

9

10

1.2

742

3

12

10

1.2

525

3

15

10

1.2

395

5

9

10

1.2

1236

5

12

10

1.2

875

5

15

10

1.2

658

5

18

10

1.2

514

ISL97645A