RT8073 Datasheet, PDF (12/16 Pages) Richtek Technology Corporation – 6A, 2MHz, High Efficiency Synchronous Step-Down Converter

RT8073
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 1.5V
is available, as shown in Figure 4. In this case, the pull-up
resistor, R_EN, is connected between V_IN and the EN pin.
MOSFET Q1 will be under logic control to pull down the
EN pin.
VOUT
(500mV/Div)
Hiccup Mode
R_EN
V_IN
EN
EN
Q1
RT8073
GND
Figure 4. Digital Enable Control Circuit
ILX
(5A/Div)
VOUT short to GND
Time (1ms/Div)
Figure 5. Hiccup Mode Under Voltage Protection
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at duty cycles greater than 50%. It is
accomplished internally by adding a compensating ramp
to the inductor current signal. Normally, the maximum
inductor peak current is reduced when slope compensation
is added. In the RT8073, however, separated inductor
current signals are used to monitor over current condition.
This keeps the maximum output current relatively constant
regardless of duty cycle.
Hiccup Mode
For the RT8073, it provides Hiccup Mode Under Voltage
Protection (UVP). When the output is shorted to ground,
the UVP function will be triggered to shut down switching
operation. If the under voltage condition remains for a
period, the RT8073 will retry automatically. When the under
voltage condition is removed, the converter will resume
operation. The UVP is disabled during soft-start period.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔI_L increases with higher V_IN
and decreases with higher inductance.
ΔI_L
=
⎡
⎢⎣
V_OUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣_1−
V_OUT
V_IN
⎤
⎥⎦
Having a lower ripple current can reduce not only the ESR
losses in the output capacitors but also the output voltage
ripple. However, it requires a large inductor to achieve this
goal.
For the ripple current selection, the value of ΔI_L = 0.4(I_MAX)
will be a reasonable starting point. The largest ripple current
occurs at the highest V_IN. To guarantee that the ripple
current stays below the specified maximum, the inductor
value should be chosen according to the following
equation :
L
=
⎡
⎢⎣
f
×
V_OUT
ΔI_L(MAX)
⎤
⎥⎦
×
⎡⎢⎣_1−
V_OUT
V_IN(MAX)
⎤
⎥⎦
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
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12
is a registered trademark of Richtek Technology Corporation.
DS8073-01 November 2012