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RT9259A

12V Synchronous Buck PWM DC-DC and Linear Power Controller

Richtek Technology 

RT9259A

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 RT9259A.

UGATE

(20V/Div)

FB

(500mV/Div)

VOUT

(20V/Div)

LGATE

(10V/Div)

UGATE

(20V/Div)

VIN1 = 12V to 0V

Time (10ms/Div)

Figure 4. UVP triggered by FB

VIN2 = 0V

RT_DIS

(500mV/Div)

UGATE

(20V/Div)

VOUT1

(500mV/Div)

LGATE

(10V/Div)

COMP

(500mV/Div)

VOUT1

(1V/Div)

Time (10ms/Div)

Figure 5. UVP hiccups triggered by FBL

Time (1ms/Div)

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 RT9259A PWM controller is shut down when VFB drops

lower than the UVP threshold. In Figure 5, the RT9259A

shuts down after 4 time UVP hiccups triggered by FBL.

Over Current Protection

The RT9259A 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 the external

resistor 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 :

IOCSET

=

40μA × ROCSET

RDS(ON)

- 0.4V

www.richtek.com

10

DS9259A-02 March 2007

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