RT6220 datasheet(13/20 Pages) RICHTEK

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RT6220

DS6220-11 July 2020

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is a registered trademark of Richtek Technology Corporation.

inductor current reaches the continuous conduction. The

transition load point to the light load operation is shown in

Figure 4 and can be calculated as follows :

Figure 4. Boundary Condition of CCM/DEM

Slope = (VIN - VOUT) / L

IPEAK

ILOAD = IPEAK / 2

tON

OUT

LOAD

)





where tON is the on-time.

The switching waveforms may appear noisy and

asynchronous when light load causes diode emulation

operation. This is normal and results in high efficiency.

Trade offs in DEM noise vs. light load efficiency is made

by varying the inductor value.Generally, low inductor values

produce a broader efficiency vs. load curve, while higher

values result in higher full load efficiency (assuming that

the coil resistance remains fixed) and less output voltage

ripple. Penalties for using higher inductor values include

larger physical size and degraded load transient response

(especially at low input voltage levels).

During discontinuous switching, the on-time is immediately

increased to add “hysteresis” to discourage the IC from

switching back to continuous switching unless the load

increases substantially. The IC returns to continuous

switching as soon as an on-time is generated before the

inductor current reaches zero. The on-time is reduced back

to the length needed for 500kHz switching and encouraging

the circuit to remain in continuous conduction, preventing

repetitive mode transitions between continuous switching

and discontinuous switching.

Linear Regulators (VCC)

The RT6220 includes a 5V linear regulator (VCC). The

VCC regulator steps down input voltage to supply both

internal circuitry and gate drivers. Do not connect the VCC

pin to external loads. When PGOOD is pulled high and

BYP pin voltage is above 4.6V, an internal 3

P-MOSFET switch connects VCC to the BYP pin while

the VCC linear regulator is simultaneously turned off.

Current Limit

The RT6220 current limit is a cycle-by-cycle “valley” type,

measuring the inductor current through the synchronous

rectifier during the off-time while the inductor current ramps

down. The current is determined by measuring the voltage

between Source and Drain of the synchronous rectifier,

adding temperature compensation for greater accuracy. If

the current exceeds the current limit, the on-time one-

shot is inhibited until the inductor current ramps down

below the current limit. If the output current exceeds the

available inductor current (controlled by the current limit

mechanism), the output voltage will drop. If it drops below

the output under-voltage protection level (see next section),

the IC will stop switching to avoid excessive heat.

Output Over-Voltage Protection and Under-Voltage

Protection

The RT6220 features an output Over-Voltage Protection

(OVP). For the RT6220A and the RT6220BL, if the output

voltage rises above the regulation level, the IC stops

switching and is latched off. On the other hand, for the

RT6220AH and the RT6220BH, the IC will stop switching

and restart automatically after a short period which is the

so-called Hiccup mode. The RT6220 also features an

output Under-Voltage Protection (UVP). If the output

voltage drops below the UVP trip threshold for longer than

μs (typical), the UVP is triggered, and the IC will

shutdown. Likewise, for the RT6220A and the RT6220BL,

the IC stops switching and is latched off. On the other

hand, for the RT6220AH and the RT6220BH, the IC will

stop switching and enter the Hiccup mode. To restart

operation from latch off, toggle EN or power the IC off and

then turn on again.

Input Under-Voltage Lockout

In addition to the enable function, the RT6220 features an

Under-Voltage Lockout (UVLO) function that monitors the

input voltage. To prevent operation without fully-enhanced

internal MOSFET switches, this function inhibits switching

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