rt9201 Richtek Technology Corporation, rt9201 Datasheet - Page 11

• Manufacturer Part Number: rt9201
• Description: High Efficiency Step-down Dc-dc Converter
• Manufacturer: Richtek Technology Corporation

Over Current Protection
The RT9201 continuously monitors the inductor current
by sensing the voltage across the PMOSFET when it
turns on. When the inductor current is higher than current
limit threshold (1A typical), OCP activates and forces the
PMOSFET turning off to limit inductor current cycle by
cycle.
The current limit is set to 50% level of normal condition
when the output voltage is lower than its 50% normal
level. This can minimize the power loss and protect the
device when over current or output short circuit occurs.
Once output voltage drops below to its 50% normal level
DS9201-04 August 2004
V
(mV)
V
(V)
OUT
LX
5.50
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
-100
Boundary of PWM and Low-Dropout Mode
100
4
2
0
0
0
Min. Input Voltage vs. Output Current
100
Sub-Harmonic at Boundary
200
Output Current (mA)
Figure 1
Figure 2
Low Dropout Mode
Time (10 s/Div)
Condition
300
PWM Mode
I
OUT
V
I
OUT
IN
(mA)
= 3.8V, V
400
= 600mA, C
V
V
V
OUT
OUT
OUT
500
OUT
= 3.3V
= 2.5V
= 1.8V
OUT
= 3.3V
600
= 100uF
Preliminary
700
due to over current or short circuit, it can build up only
when output current is lower 50% of normal current limit
threshold.
Output Voltage Setting and Feedback Network
The output voltage can be set from V
voltage divider as:
The internal V
application, keep R
respectively and choose appropriate R
according to the required output voltage. Make sure that
the product of R
following table shows recommended feedback network
as well as inductor for some typical output voltages.
Table 1. Component Selection for Typical Application
Inductor Selection
The output inductor is suggested as the above table for
optimal performance. Make sure that the inductor will
not saturate over the operation conditions including
temperature range, input voltage range, and maximum
output current. If possible, choose an inductor with rated
current higher than 1A so that it will not saturate even
under circuit condition.
Input Capacitor Selection
The input capacitor can filter the input peak current and
noise at input voltage source. The capacitor with low ESR
(effective series resistance) provides the small drop
voltage to stabilize the input voltage during the transient
loading. For input capacitor selection, the ceramic
capacitors larger than 10 F is recommend. The capacitor
must conform to the RMS current requirement. The
maximum RMS ripple current is calculated as:
V
0.85
1.5
2.5
3.3
(V)
OUT
I
V
RMS
OUT
1.2M
343k
820k
R
24k
( )
I
OUT(MAX)
V
F1
REF
REF
F1
is 0.8V with 1% accuracy. In practical
(pF)
C
330
R
5.6
22
10
and C
F1
F1
R
V
F2
F2
OUT
R
= 390k
F2
390k
390k
390k
390k
( )
F1
R
(V
F2
V
is about 8200k -pF. The
IN
IN
V
(pF)
C
10
10
10
10
OUT
F2
and C
RT9201
REF
)
www.richtek.com
( H)
4.7
6.8
L
10
10
F1
to V
F2
X
and C
= 10pF
IN
1000
C
(pF)
330
470
680
by a
SS
11
F1