6
RT8525
www.richtek.com
DS8525-01 March 2012
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Applications Information
The RT8525 is a wide input operating voltage range step
up controller. High voltage output and large output current
are feasible by using an external N-MOSFET. The
protection functions include output over voltage, output
under voltage, over temperature and current limiting
protection.
Boost Output Voltage Setting
The regulated output voltage is set by an external resistor
divider according to the following equation :
⎛⎞
×
⎜⎟
⎝⎠
FB1
OUT FB FB
FB2
R
V = V 1+ , where V = 1.25V (typ.)
R
The recommended value of R
FB2
should be at least 1kΩ
for saving sacrificing. Moreover, placing the resistor divider
as close as possible to the chip can reduce noise
sensitivity.
Boost Switching Frequency
The RT8525 boost driver switching frequency is able to
be adjusted by a resistor R
SW
ranging from 18kΩ to
220kΩ. The following figure illustrates the corresponding
switching frequency within the resistor range.
Figure 1. Boost Switching Frequency
Switching Frequency vs. R
SW
0
100
200
300
400
500
600
0 50 100 150 200 250
R
SW
(k )
f
SW
(kHz)
Ω
Boost Loop Compensation
The voltage feedback loop can be compensated by an
external compensation network consisted of R
C
, C
C1
and
C
C2
. Choose R
C
to set high frequency gain for fast
transient response. Select C
C1
and C
C2
to set the zero
and pole to maintain loop stability. For typical application,
V
IN
= 24V, V
OUT
= 50V, C
OUT
= 100μF x 2, L1 = 33μH,
while the recommended value for compensation is as
follows : R
C
= 33kΩ, C
C1
= 27nF.
Soft-Start
The soft-start of the RT8525 can be achieved by connecting
a capacitor from the SS pin to GND. The built-in soft-start
circuit reduces the start-up current spike and output
voltage overshoot. The external capacitor charged by an
internal 4μA constant charging current determines the soft-
start time. The SS pin limits the rising rate of the COMP
pin voltage and thereby limits the peak switch current.
The soft-start interval is set by the soft-start capacitor
according to the following equation :
≅××
5
SS SS
tC510
A typical value for the soft-start capacitor is 0.33μF. The
soft-start capacitor is discharged when EN voltage falls
below its threshold after shutdown delay or UVLO occurs.
Slope Compensation and Current Limiting
A slope compensation is applied to avoid sub-harmonic
oscillation in current-mode control. The slope
compensation voltage is generated by the internal ramp
current flow through a slope compensation resistor R
SLP
.
The inductor current is sensed by the sensing resistor
R
S
. Both of them are added and presented on the ISW
pin. The internal ramp current is rising linearly form zero
at the beginning of each switching cycle to 50μA in
maximum on-time of each cycle. The slope compensation
resistor R
SLP
can be calculated by the following equation :
where R
S
is current sensing resistor, L is inductor value,
and f
SW
is boost switching frequency.
The current flow through inductor during charging period
is detected by a sensing resistor R
S
. Besides, the slope
compensation voltage also attributes magnitude to ISW.
As the voltage at the ISW pin is over 0.4V, the DRV will
be pulled low and turn off the external N-MOSFET. So
that the inductor will be forced to leave charging stage
and enter discharging stage to prevent over current. The
current limiting can be calculated by the following equation:
()
−×
×× ×
OUT IN S
SLP
SW
VVR
R >
2L50μ f
