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RT8525D Datasheet(PDF) 8 Page - Richtek Technology Corporation

Part # RT8525D
Description  Current Mode Boost Controller
Download  12 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT8525D Datasheet(HTML) 8 Page - Richtek Technology Corporation

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8
RT8525D
www.richtek.com
DS8525D-00 June 2012
©
Copyright 2012 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
Applications Information
The RT8525D 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 RFB2 should be at least 1k
Ω
for saving sacrificing. Moreover, placing the resistor divider
as close as possible to the chip can reduce noise
sensitivity.
Boost Switching Frequency
The RT8525D boost driver switching frequency is able to
be adjusted by a resistor RSW ranging from 18k
Ω to
220k
Ω. The following figure illustrates the corresponding
switching frequency within the resistor range.
Figure 1. Boost Switching Frequency
Switching Frequency vs. RSW
0
100
200
300
400
500
600
0
50
100
150
200
250
RSW (k )
Ω
Boost Loop Compensation
The voltage feedback loop can be compensated by an
external compensation network consisted of RC, CC1 and
CC2. Choose RC to set high frequency gain for fast
transient response. Select CC1 and CC2 to set the zero
and pole to maintain loop stability. A typical compensation
for the RT8525D is choosing 3k
Ω for RC and 27nF for CC1.
Soft-Start
The soft-start of the RT8525D 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 4
μA 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
tC
5 10
A typical value for the soft-start capacitor is 0.33
μF. 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 RSLP.
The inductor current is sensed by the sensing resistor
RS. 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 50
μA in
maximum on-time of each cycle. The slope compensation
resistor RSLP can be calculated by the following equation :
where RS is current sensing resistor, L is inductor value,
and fSW is boost switching frequency.
The current flow through inductor during charging period
is detected by a sensing resistor RS. 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
VV
R
R
>
2L 50μ f


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