RT2875A/B
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section.
Current Setting
The current limit of high side MOSFET is adjustable by
an external resistor connected to the RLIM pin. The current
limit range is from 1.5A to 6A. When the inductor current
reaches the current limit threshold, the COMP voltage
will be clamped to limit the inductor current. Inductor
current ripple current also should be considered into
current limit setting. Current limit minimum value should
be set as below :
The output ripple, ΔVOUT , is determined by :
1
VOUT I ESR
L
8fCOUT
The output ripple will be the highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Higher values,
lower cost ceramic capacitors are now becoming available
in smaller case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. However, care must be taken when these
capacitors are used at input and output. When a ceramic
capacitor is used at the input and the power is supplied
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at
VIN large enough to damage the part.
Current limit minimum = (IO(max) + 1 / 2 inductor current
ripple) x 1.2
Through extra resister RLIMconnect to RLIM pin to setting
the current limit value below offer approximate formula
equation :
ISET = current limit value (A)
y = (ISET − 0.4206) / 167.79
RLIM (kΩ) = (1 / y)
Soft-Start
The RT2875A/B provides soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up. The soft-start timing can
be programmed by the external capacitor CSS between
SS andGND.An internal current source ISS (6μA) charges
an external capacitor to build a soft-start ramp voltage.
The VFB voltage will track the internal ramp voltage during
softstart interval. The typical soft start time is calculated
as follows :
Switching Frequency Setting
The switching frequency can be set by using extra resister
RT or external clock. Switching frequency range is from
300kHz to 2.1MHz. Through extra resister RT connect to
RT/SYNC pin to setting the switching frequency FS, below
offer approximate formula equation :
Soft-Start time tSS = CSS x 0.6 / 6μA
Setting Frequency = FS (kHz)
x = [FS − 31.379] / 47691
ROSC (kΩ) = (1 / x)
The RT2875A/B can be synchronized with an external clock
ranging from 300kHz to 2.1MHz applied to the RT/SYNC
pin. The external clock duty cycle must be from 10% to
90%. The RT/SYNC pin is at logic-high level (>2V). If the
EN pin is pulled to low-level for 10μs above, the IC will
shut down.
Copyright 2016 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
14
DS2875A/B-05 October 2016
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
