RAA207703GBM/7704GBM/7705GBM

R07DS0892EJ0100 Rev.1.00

Page 10 of 23

Aug 02, 2013

Description of Operation

The RAA207703GBM operates as voltage-ripple based constant on time control architecture. Converter output is

controlled by output voltage ripple which is determined by inductor ripple current and ESR & ESL of output capacitor.

Each switching cycle starts High-side MOSFET turn on which time is decided by 1 shot timer. After High-side

MOSFET turns off, Low side turns on, and it keeps until FB voltage becomes lower than reference voltage. In light load

condition, Low-side MOSFET on time is decided by inductor zero current.

Switching Frequency, Constant on Time Setting

SW

SET

Rset

Switching Frequency in CCM mode is determined by following equation.

Switching Frequency: (Vout / VIN) • (1 / ton) [Hz]

¼(1)

Here, ton is High-side MOSFET on time, and it is determined by following equation.

On time pulse: (50 pF • 1 V / (VIN – 2.0 V)) • Rset + 60 ns [s]

¼(2)

From above equation, constant on time is change depend on VIN, so switching frequency is almost constant when VIN

change. This architecture is suitable for battery application. From the above equation, Rset is calculated by

Rset: (Vout / (VIN • Fsw) – 60 ns) • (VIN – 2.0 V) / (50 pF • 1 V) [

W] ¼(3)

Here, Fsw is switching frequency.

100

200

300

400

500

600

700

800

900

1000

10

30

50

70

90

20

40

60

80

100

Rset [k

W]

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

4

6

8

10

12

14

16

VIN [V]

VIN = 5 V

VIN = 8.5 V

VIN = 12 V

Rset = 51 k

W

Rset = 30 k

W

Rset = 10 k

W

Minimum on time is 70 ns (typ.), so recommended on time pulse is more than 100 ns. Maximum operating frequency is

restricted by minimum on time and minimum off time (50 ns, please see next chapter).