RT7274 datasheet(7/22 Pages) RICHTEK

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RT7274/79/80/81

DS7274/79/80/81-01 February 2013

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is a registered trademark of Richtek Technology Corporation.

Input Under-voltage Lock-out

In addition to the enable function, the RT7274/79/80/81

feature an under-voltage lock-out (UVLO) function that

monitors the internal linear regulator output (PVCC). To

prevent operation without fully-enhanced internal MOSFET

switches, this function inhibits switching when PVCC

drops below the UVLO-falling threshold. The IC resumes

switching when PVCC exceeds the UVLO-rising threshold.

Soft-Start (SS)

The RT7274/79/80/81 soft-start uses an external pin (SS)

to clamp the output voltage and allow it to slowly rise.

After VEN is high and PVCC exceeds its UVLO threshold,

the IC begins to source 2

μA from the SS pin. An external

capacitor at SS is used to adjust the soft-start timing.

The available capacitance range is from 2.7nF to 220nF.

Do not leave SS unconnected.

During start-up, while the SS capacitor charges, the

RT7274/79/80/81 operate in discontinuous switching mode

with very small pulses. This prevents negative inductor

currents and keeps the circuit from sinking current.

Therefore, the output voltage may be pre-biased to some

positive level before start-up. Once the VSS ramp charges

enough to raise the internal reference above the feedback

voltage, switching will begin and the output voltage will

smoothly rise from the pre-biased level to its regulated

level. After VSS rises above about 2.2V output over-and

under-voltage protections are enabled and the RT7279/81

begins continuous-switching operation.

Internal Regulator (PVCC)

An internal linear regulator (PVCC) produces a 5.1V supply

from VIN that powers the internal gate drivers, PWM logic,

reference, analog circuitry, and other blocks. If VIN is 6V

or greater, PVCC is guaranteed to provide significant power

for external loads.

PGOOD Comparator

PGOOD is an open drain output controlled by a comparator

connected to the feedback signal. If FB exceeds 90% of

the internal reference voltage, PGOOD will be high

impedance. Otherwise, the PGOOD output is connected

to PGND.

high, the synchronous rectifier turns off until after the next

high-side on-time. RT7274/80 does not sink current and

therefore does not need a negative current limit.

Output Over-voltage Protection and Under-voltage

Protection

The RT7279/80 include output over-voltage protection

(OVP). If the output voltage rises above the regulation

level, the high-side switch naturally remains off and the

synchronous rectifier turns on. If the output voltage remains

high the synchronous rectifier remains on until the inductor

current reaches the negative current limit (RT7279) or until

it reaches zero (RT7280). If the output voltage remains

high, the IC's switches remain off. If the output voltage

exceeds the OVP trip threshold for longer than 5

μs

(typical), the IC's OVP is triggered.

The RT7279/80 include output under-voltage protection

(UVP). If the output voltage drops below the UVP trip

threshold for longer than 250

μs (typical) the IC's UVP is

triggered.

There are two different behaviors for OVP and UVP events

for the TSSOP-14 (Exposed Pad) packages.

Latch-Off Mode (TSSOP-14 (Exposed Pad) Only)

The RT7280GCP/RT7279GCP, use latch-off mode OVP

and UVP. When the protection function is triggered the

IC will shut down. The IC stops switching, leaving both

switches open, and is latched off. To restart operation,

toggle EN or power the IC off and then on again.

Shut-down, Start-up and Enable (EN)

The enable input (EN) has a logic-low level of 0.4V. When

VEN is below this level the IC enters shutdown mode and

supply current drops to less than 10

μA. When VEN exceeds

its logic-high level of 1.6V the IC is fully operational.

Between these 2 levels there are 2 thresholds (1.2V typical

and 1.4V typical). When VEN exceeds the lower threshold

the internal bias regulators begin to function and supply

current increases above the shutdown current level.

Switching operation begins when VEN exceeds the upper

threshold. Unlike many competing devices, EN is a high

voltage input that can be safely connected to VIN (up to

18V) for automatic start-up.

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