RT9259CGS datasheet(9/15 Pages) RICHTEK

9 / 15 page

RT9259C

DS9259C-03

August 2007

www.richtek.com

Soft Start for Synchronous Buck Converter

A built-in soft-start is used to prevent surge current from

power supply input during power on (referring to the

Functional Block Diagram). The error amplifier EA is a three-

input device. SSE or VREF1 whichever is smaller dominates

the behavior non-inverting input. The internal soft start

voltage SSE linearly ramps up to about 4V after VIN1

existence is recognized with about 2ms delay. According,

the output voltage ramps up smoothly to its target level.

The rise time of output voltage is about 2ms as shown in

Figure 3. VREF1 takes over the behavior EA when SSE >

VREF1.

SSE is also used for LDO soft start. LDO input voltage

VIN2 MUST be ready before SSE starts to ramp up.

Otherwise UVP function of LDO may be triggered and shut

down the RT9259C.

Figure 3 : Start Up by RT_DIS

Under Voltage Protection

The voltages at FB and FBL pin are monitored for under

voltage protection (UVP) after the soft start is completed.

UVP is triggered if one of the feedback voltages is under

(50% x VREFX) with a 30us delay. As shown in Figure 4,

the RT9259C PWM controller is shut down when VFB drops

lower than the UVP threshold. In Figure 5, the RT9259C

shuts down after 4 time UVP hiccups triggered by FBL.

Figure 4. UVP triggered by FB

Figure 5. UVP Hiccups Triggered by FBL

Time (1ms/Div)

RT_DIS

(500mV/Div)

UGATE

(20V/Div)

VOUT1

(500mV/Div)

LGATE

(10V/Div)

Over Current Protection

The RT9259C senses the current flowing through lower

MOSFET for over current protection (OCP) by sensing the

PHASE pin voltage as shown in the Functional Block

Diagram. A 40uA current source flows through internal 20k

ROCSET to PHASE pin causes 0.8V voltage drop across

the resistor. OCP is triggered if the voltage at PHASE pin

(drop of lower MOSFET VDS) is lower than

−0.4V when low

side MOSFET conducting. Accordingly inductor current

threshold for OCP is a function of conducting resistance

of lower MOSFET RDS(ON) as :

DS(ON)

OCSET

0.4V

)

(20k

Time (10ms/Div)

(500mV/Div)

UGATE

(20V/Div)

VOUT

(20V/Div)

VIN1 = 12V to 0V

Time (10ms/Div)

LGATE

(10V/Div)

VOUT1

(1V/Div)

UGATE

(20V/Div)

COMP

(500mV/Div)

VIN2 = 0V

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