rt8070 Richtek Technology Corporation, rt8070 Datasheet - Page 8

• Manufacturer Part Number: rt8070
• Description: 4a, 2mhz, Synchronous Step-down Converter
• Manufacturer: Richtek Technology Corporation

RT8070
Application Information
The basic IC application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by C
Main Control Loop
During normal operation, the internal upper power switch
(P-MOSFET) is turned on at the beginning of each clock
cycle. Current in the inductor increases until the peak
inductor current reaches the value defined by the output
voltage (V
adjusts its output voltage by comparing the feedback signal
from a resistive voltage-divider on the FB pin with an
internal 0.8V reference. When the load current increases,
it causes a reduction in the feedback voltage relative to
the reference. The error amplifier increases its output
voltage until the average inductor current matches the new
load current. When the upper power MOSFET shuts off,
the lower synchronous power switch (N-MOSFET) turns
on until the beginning of the next clock cycle.
Output Voltage Setting
The output voltage is set by an external resistive voltage-
divider according to the following equation :
where V
The resistive voltage-divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
Soft-Start
The IC contains an external soft-start clamp that gradually
raises the output voltage. The soft-start timing is
programmed by the external capacitor between SS pin
and GND. The chip provides an internal 10μA charge current
www.richtek.com
8
V
OUT
= V
REF
REF
Figure 1. Setting the Output Voltage
COMP
equals to 0.8V typical.
IN
×
) of the error amplifier. The error amplifier
and C
(1+
R2
R1
RT8070
OUT
)
.
GND
FB
V
OUT
R1
R2
for the external capacitor. If 10nF capacitor is used to set
the soft-start, the period will be 800μs (typ.).
Power Good Output
The power good output is an open-drain output and requires
a pull up resistor. When the output voltage is 12.5% above
or 12.5% below its set voltage, PGOOD will be pulled
low. It is held low until the output voltage returns to within
the allowed tolerances once more. During soft-start,
PGOOD is actively held low and is only allowed to transition
high when soft-start is over and the output voltage reaches
87.5% of its set voltage.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. Higher frequency operation
allows the use of smaller inductor and capacitor values.
Lower frequency operation improves efficiency by reducing
internal gate charge and switching losses but requires
larger inductance and/or capacitance to maintain low output
ripple voltage.The operating frequency of the IC is
determined by an external resistor , R
connected between the RT pin and ground. The value of
the resistor sets the ramp current that is used to charge
and discharge an internal timing capacitor within the
oscillator. The practical switching frequency ranges from
200kHz to 2MHz. However, when the RT pin is floating,
the internal frequency is set at 2MHz. Determine the RT
resistor value by examining the curve below.
Figure 2. Switching Frequency vs. R
2.4
2.0
1.6
1.2
0.8
0.4
0.0
0
300
600
900
R
RT
(k )
DS8070-03 March 2011
1200
Ω
1500
RT
Resistor
OSC
1800
, that is
2100