RT8105 查看數據表（PDF） - Richtek Technology

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RT8105

5V/12V Synchronous Buck PWM DC/DC Controller

Richtek Technology 

RT8105

try to restart in a hiccupped way. Figure 3 shows the

hiccupped over current protection. Only four times of

hiccup is allowed in over current protection. If over current

condition still exist after four times of hiccup, controller

will be latched.

COUNT = 1

4V

2V

0V

COUNT = 2 COUNT = 3

OVERLOAD

APPLIED

COUNT = 4

0A

T0 T1

T2

T3

T4

TIME

Figure 3. Hiccupped Over Current Protection

Over Voltage Protection (OVP)

The feedback voltage is continuously monitored for over

voltage protection. When OVP is tripped, LGATE will go

high and UGATE will go low to discharge the output

capacitor.

RT8105 provides full-time over voltage protection whenever

soft start completes or not.

Over voltage protection has two operating conditions:

before soft start completes and after soft start completes.

Each condition is described as follows.

Before soft start completes, the typical OVP threshold is

137.5% of the internal reference voltage VREF. RT8105

provides non-latched OVP before soft start completes.

The controller will return to normal operation if over voltage

condition is removed.

After soft start completes, however, the OVP threshold is

typically 162.5% of VREF. RT8105 provides latched OVP

after soft start completes. The controller can only be reset

if VCC POR is exceeded again.

Under Voltage Protection (UVP)

The feedback voltage is also monitored for under voltage

protection. The under voltage protection has 15us triggered

delay. When UVP is tripped, both UGATE and LGATE will

go low. Unlike OCP, UVP is not a latched protection;

controller will always try to restart in a hiccupped way.

Enable/Disable

The controller can be disabled by pulling OPS pin to

ground. The enable/disable function can be implemented

by connecting a MOSFET or BJT to OPS pin. It is

recommended to use small signal MOSFET/BJT to

implement the enable/disable function.

Output Inductor Selection

The selection of output inductor depends on the efficiency,

output current and operating frequency. Low inductance

value can have fast transient response, but the associated

large current ripple will cause large output ripple voltage

and decrease the efficiency.

In general, a 20% to 40% of inductor ripple current

percentage (ΔIL / IOUT) is preferred in practical application.

The minimum inductance can be determined as follows :

L

=

(VIN

−

VOUT

)×

VIN

VOUT

× fS ×

ΔIL

Where :

VIN = Input voltage

VOUT = Output voltage

ΔIL = Inductor current ripple

fS = Switching frequency

Output Capacitor Selection

The selection of output capacitor depends on the inductor

ripple current, the output ripple voltage and the amount of

voltage under shoot during transient. The output ripple

voltage is a function of both the capacitance and the

equivalent series resistance (ESR) rC. The output ripple

voltage can be expressed as follows :

ΔVOUT = ΔVOR + ΔVOC

ΔVOUT

=

ΔIL

× rc

+

1

CO

∫ t2

t1

ic

dt

ΔVOUT

=

ΔIL

× ΔIL

× rc

+

1

8

VOUT

COL

(1− D)T 2

S

where ΔVOR is caused by ESR, and ΔVOC is related to the

capacitance value.

For electrolytic capacitor application, major of the output

voltage ripple is typically contributed by the ESR.

Therefore, the output voltage ripple can be simplified as

follows :

ΔVOUT = ΔIL x rC

DS8105-03 April 2011

www.richtek.com

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