DS6550A Datasheet, PDF (11/16 Pages) Richtek Technology Corporation – Synchronous DC-DC Step-Down Controller with 5V LDO

PHASE is above the current limit threshold, the PWM is
not allowed to initiate a new cycle (Figure 2). The actual
peak current is greater than the current limit threshold by
an amount equal to the inductor ripple current. Therefore,
the exact current limit characteristic and maximum load
capability are a function of the sense resistance, inductor
value, battery and output voltage.
I_L
I_PEAK
I_LOAD
I_LIMIT
t
Figure 2. “Valley” Current Limit
The RT6550A uses the on resistance of the synchronous
rectifier as the current sense element and supports
temperature compensated MOSFET R_DS(ON) sensing. The
R_ILIM resistor between the CS pin and GND sets the current
limit threshold. The resistor R_ILIM is connected to a current
source from CS which is 10μA (typ.) at room temperature.
The current source has a 4700ppm/°C temperature slope
to compensate the temperature dependency of the
R_DS(ON). When the voltage drop across the sense resistor
or low-side MOSFET equals 1/8 the voltage across the
R_ILIM resistor, positive current limit will be activated. The
high-side MOSFET will not be turned on until the voltage
drop across the MOSFET falls below 1/8 the voltage across
the R_ILIM resistor.
Choose a current limit resistor according to the following
equation :
V_LIMIT = (R_LIMIT x 10μA) / 8 = I_LIMIT x R_DS(ON)
R_LIMIT = (I_LIMIT x R_DS(ON)) x 8 / 10μA
Carefully observe the PC board layout guidelines to ensure
that noise and DC errors do not corrupt the current sense
signal at PHASE and GND. Mount or place the IC close
to the low-side MOSFET.
RT6550A
MOSFET Gate Driver (UGATE, LGATE)
The high-side driver is designed to drive high current, low
R_DS(ON) N-MOSFET(s). When configured as a floating driver,
5V bias voltage is delivered from the LDO5 supply. The
average drive current is also calculated by the gate charge
at V_GS = 5V times switching frequency. The instantaneous
drive current is supplied by the flying capacitor between
the BOOT and PHASE pins. A dead-time to prevent shoot
through is internally generated from high-side MOSFET
off to low-side MOSFET on and low-side MOSFET off to
high-side MOSFET on.
The low-side driver is designed to drive high current low
R_DS(ON) N-MOSFET(s). The internal pull down transistor
that drives LGATE low is robust, with a 1Ω typical on-
resistance. A 5V bias voltage is delivered from the LDO5
supply. The instantaneous drive current is supplied by an
input capacitor connected between LDO5 and GND.
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,
and shoot through currents. This is often remedied by
adding a resistor in series with BOOT, which increases
the turn-on time of the high-side MOSFET without
degrading the turn-off time. See Figure 3.
V_IN
UGATE
BOOT
R_BOOT
PHASE
Figure 3. Increasing the UGATE Rise Time
Soft-Start
The RT6550A provides an internal soft-start function to
prevent large inrush current and output voltage overshoot
when the converter starts up. The soft-start (SS)
automatically begins once the chip is enabled. During soft-
start, it clamps the ramping of internal reference voltage
which is compared with FB signal. The typical soft-start
duration is 0.9ms. An unique PWM duty limit control that
prevents output over-voltage during soft-start period is
designed specifically for FB floating.
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS6550A-00 September 2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11