RT8058 Ver la hoja de datos (PDF) - Richtek Technology
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RT8058 Datasheet PDF : 15 Pages
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RT8058
Preliminary
Application Information
Function Description
The RT8058 is a 1MHz constant frequency, current mode
PWM step-down converter. High switching frequency and
high efficiency make it suitable for applications where high
efficiency and small size are critical.
Frequency compensation is done internally. The output
voltages are set by external dividers returned to the FB
pin. The output voltage can be set from 0.8V to 5V.
Main Control Loop
During normal operation, the internal top power switch
(P-MOSFET) is turned on at the beginning of each clock
cycle. Current in the inductor increases until the peak
inductor current reach the value defined by the output
voltage of the error amplifier. The error amplifier adjusts its
output voltage by comparing the feedback signal from a
resistor divider on the FB pin with an internal 0.6V
reference. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier raises its output voltage until the average
inductor current matches the new load current. When the
top power MOSFET shuts off, the synchronous power
switch (N-MOSFET) turns on until the beginning of the
next clock cycle.
Soft Start / Enable
For convenience of power up sequence control, RT8058
has an enable pin. Logic high at EN pin will enable the
converter. When the converter is enabled, the clamped
error amplifier output ramps up during 1024-clock period
to increase the current provided by converter until the
output voltage reach the target voltage. If EN is kept at
high during Vin applying, RT8058 will be enabled when
VDD surpass Under-Voltage Lockout threshold.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation :
VOUT = VREF x (1+ R1/R2)
where VREF equals to 0.6V typical.
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8058
R2
GND
Figure 1. Setting the Output Voltage
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 RT8058, however, separated inductor current
signal is used to monitor over current condition and this
keeps the maximum output current relatively constant
regardless of duty cycle.
Dropout Operation
When input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on time. Further reduction of the supply voltage forces
the main switch to remain on for more than one cycle
eventually reaching 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the internal P-MOSFET and the
inductor.
Low Supply Operation
The RT8058 is designed to operate down to an input supply
voltage of 2.7V. One important consideration at low input
supply voltages is that the RDS(ON) of the P-Channel and
N-Channel power switches increases. The user should
calculate the power dissipation when the RT8058 is used
at 100% duty cycle with low input voltages to ensure that
thermal limits are not exceeded.
www.richtek.com
10
DS8058-02 August 2007
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