RT6908 datasheet(18/22 Pages) RICHTEK

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RT6908

DS6908-01

March 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

VI/O Buck Soft-Start

The step-down converter has an internal soft-start to reduce

the input inrush current. When the buck converter is

enabled, the output voltage rises slowly from zero to the

regulated voltage. The typical soft-start time is around

300

μs.

VI/O Buck Over Current Protection

The IC senses the inductor current that is flowing out the

LXB1 pin. The internal MOSFET will be turned off if the

peak inductor current reaches 3.8A (typ.).

VI/O Buck Short Circuit Protection

To limit the short circuit current, the device has a cycle-

by-cycle current limit. To avoid the short circuit current

from rising above the internal current limit when the output

is shorted to GND, the switching frequency is reduced as

well. The switching frequency is reduced to one-half of

original frequency when the output voltage is below 80%

and to one-fourth of the original frequency when the output

voltage is below 20%. If the

“short” is removed, the buck

converter will resume operation. If the voltage remains

below 80% after 50ms, the IC will shut down.

VI/O Buck Loop Compensation

The voltage feedback loop can be compensated with an

external compensation network consisted of the R4 and

C19. Choose R4 to set high frequency integrator gain for

fast transient response. And choose the C19 to set the

integrator zero to maintain stability.

VCORE Synchronous Buck Controller

The synchronous buck controller is a high efficiency PWM

architecture with 500kHz operation frequency and fast

transient response. The controller need external high side

and low side N-MOSFET as synchronous rectifier and does

not required Schottky diode on the LXB2 pin. The high

side MOSFET is connected between VIN and the LXB2

pin, while the low side MOSFET is connected between

the LXB2 pin and GND.

VCORE Buck Output Voltage Setting

The regulated default output voltage as shown in the

following equation :

The recommended value for R11 should be up to 10k

without some sacrificing. Place the resistor divider as

close as possible to the chip can reduce noise sensitivity.

The output voltage also can be adjusted from

−7% to 7%

by setting the I2C register 04h [3:0].

VCORE Buck Soft-Start

The synchronous buck converter has an internal soft-start

to reduce the input inrush current. When the converter is

enabled, the output voltage rises slowly from zero to the

regulated voltage. The typical soft-start time is around

300

μs.

VCORE Buck Over Current Protection

The IC has a cycle-by-cycle low side source current

sensing algorithm that uses the on-resistance of the low

side MOSFET as a current sensing element, so that costly

sense resistors are not required. The DHB2 and DLB2 will

be turned off if the low side MOSFET source current

reaches setting value. Moreover, IC will restart after 50ms

if over current remains 7 cycles. Low side source peak

current limit threshold is 1/8 voltage at the ILIMT2 pin.

Meanwhile, the real current limit value need consider the

on-resistance of the low side MOSFET.

⎛⎞

×⎜⎟

⎝⎠

CORE

FBB2

R11

= V

1 +

, where V

= 0.8V (typ.)

R12

ILIMIT2

LIM_M2

(A)

(switch)

when ILIM_M2 is “Low side switch current limit”.

VCORE Buck Short Circuit Protection

To limit the short circuit current, the device has a cycle-

by-cycle current limit. To avoid the short circuit current

from low side MOSFET source current limit when the

output is shorted to GND, the switching operation will be

stop. The switching operation will stop when the output

voltage is below 20%. If the short is removed, the buck

converter will resume operation. If the voltage remains

below 80% after 50ms, the IC will shut down.

VCORE Buck Loop Compensation

The voltage feedback loop can be compensated with an

external compensation network consisted of R23 and C25

and C26. Choose R23 to set high frequency integrator

gain for fast transient response, C25 and C26 to set the

integrator zero to maintain stability.

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