RT8207LZQW datasheet(20/27 Pages) RICHTEK

20 / 27 page

DS8207L/M-08 September 2016

www.richtek.com

RT8207L/M

is a registered trademark of Richtek Technology Corporation.

The low side driver is designed to drive high current, low

RDS(ON) N-MOSFET(s). The internal pull down transistor

that drives LGATE low is robust, with a 0.8

Ω typical on

resistance. A 5V bias voltage is delivered from the VDDP

supply. The instantaneous drive current is supplied by the

flying capacitor between VDDP and PGND.

For high current applications, some combinations of high

and low side MOSFETs may cause excessive gate drain

coupling, which leads to efficiency killing, EMI producing

shoot through currents. This is often remedied by adding

a resistor in series with BOOT, which increases the turn-

on rising time of the high side MOSFET without degrading

the turn-off time (Figure 7).

BOOT

UGATE

PHASE

VIN

Figure 7. Increasing the UGATE Rise Time

Power Good Output (PGOOD)

The power good output is an open drain output that requires

a pull up resistor. When the output voltage is 15% above

or 10% below its set voltage, PGOOD gets pulled low. It

is held low until the output voltage returns to within these

tolerances once more. During soft-start, PGOOD is actively

held low and only allowed to transition high after soft-start

is over and the output reaches 93% of its set voltage.

There is a 2.5

μs delay built into PGOOD circuitry to prevent

false transition.

POR Protection

The RT8207L/M has a VDDP supply power on reset

protection (POR). When the VDDP voltage is higher than

4.2V (typ.), VDDQ, VTT and VTTREF will be activated.

This is a non-latch protection.

Soft-Start

The RT8207L/M provides an internal soft-start function to

prevent large inrush current and output voltage overshoot

when the converter starts up. Soft-start (SS) automatically

begins once the chip is enabled. During soft-start, internal

current limit circuit gradually ramps up the inductor current

from zero. The maximum current-limit value is set

externally as described in previous section. The soft-start

time is determined by the current limit level and output

capacitor value. If the current limit threshold is set for

200mV, the typical soft-start duration is 3ms after S5 is

enabled.

The soft-start function of VTT is achieved by the current

limit and VTTREF voltage through the internal RC delay

ramp up after S3 is high. During VTT startup, the current

limit level is 2.6A. This allows the output to start up

smoothly and safely under enough source/sink ability.

Output Over Voltage Protection (OVP)

The output voltage can be continuously monitored for over

voltage. If the output exceeds 15% of its set voltage

threshold, over voltage protection is triggered and the

LGATE low side gate driver is forced high. This activates

the low side MOSFET switch which rapidly discharges

the output capacitor and reduces the input voltage. There

is a 5

μs latch delay built into the over voltage protection

circuit. The RT8207L/M will be latched if the output voltage

remains above the OV threshold after the latch delay

period and can then only be released by VDD power on

reset or S5.

Note that latching the LGATE high will cause the output

voltage to dip slightly negative when energy has been

previously stored in the LC tank circuit. For loads that

cannot tolerate a negative voltage, place a power Schottky

diode across the output to act as a reverse polarity clamp.

If the over voltage condition is caused by a short in high

side switch, turning the low side MOSFET on 100% will

create an electrical short between the battery and GND,

hence blowing the fuse and disconnecting the battery from

the output.

Output Under Voltage Protection (UVP)

The output voltage can be continuously monitored for under

voltage. When enabled, the under voltage protection is

triggered if the output is less than 70% of its set voltage

threshold. Then, both UGATE and LGATE gate drivers will

be forced low while entering soft discharge mode. During

soft-start, the UVP has a blanking time around 5ms.

RT8207LZQW Datasheet(PDF) 20 Page - Richtek Technology Corporation

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