n
TYPICAL APPLICATIONS AND APPLICATION HINTS
PMOS
L
V
IN
CE
C
IN
GND
EXT
V
OUT
SD1
C
OUT
Load
CE CONTROL
PMOS : HAT1020R(Hitachi), Si3443DV(Siliconix)
SD1
C
IN
: RB491D (Rohm)
: 10mF52(Tantalum Type)
L
: CD105(Sumida, 27mH)
C
OUT
: 47mF(Tantalum Type)
When you use these ICs, consider the following issues;
l
As shown in the block diagram, a parasitic diode is formed in each terminal, each of these diodes is not formed
for load current, therefore do not use it in such a way. When you control the CE pin by another power supply,
do not make its "H" level more than the voltage level of V
IN
pin.
l
The operation of Latch-type protection circuit is as follows;
When the maximum duty cycle continues longer than the delay time for protection circuit, (Refer to the Electrical
Characteristics) the protection circuit works to shut-down Power MOSFET with its latching operation. Therefore
when an input/output voltage difference is small, the protection circuit may work with small load current.
To release the protection latch state, after disable this IC with a chip enable circuit, enable it again, or restart this
IC with power-on. However, in the case of restarting this IC with power-on, after the power supply is turned off, if a
certain amount of charge remains in C
IN,
or some voltage is forced to V
IN
from C
IN,
this IC might not be restarted
even after power-on.
If rising transition speed of supply voltage is too slow, or the time which is required for V
IN
voltage to reach Output
voltage of DC/DC converter is longer than soft-starting time plus delay time for protection circuit, protection circuit
works before V
IN
voltage reaches Output voltage of DC/DC converter. To prevent this action, while power supply
voltage is not ready, make this IC be standby mode(CE=”L”), and when the power supply is ready (the voltage level
of V
IN
is equal or more than the voltage level of V
OUT
), make it enable(CE=”H”).
l
The operation of Reset-type protection circuit is as follows;
When the maximum duty cycle continues longer than the delay time for protection circuit, (Refer to the Electrical
Characteristics) the protection circuit works to restart with soft-start operation. Therefore when an input/output
voltage difference is small, the protection circuit may work with small load current.
l
Set external components as close as possible to the IC and minimize the connection between the components
and the IC. In particular, a capacitor should be connected to V
OUT
pin with the minimum connection. And make
sufficient grounding and reinforce supplying. A large switching current flows through the connection of power
supply, an inductor and the connection of V
OUT.
If the impedance of the connection of power supply is high, the
voltage level of power supply of the IC fluctuates with the switching current. This may cause unstable operation of
the IC.
l
Use capacitors with a capacity of 22mF or more for V
OUT
pin, and with good high frequency characteristics such
as tantalum capacitors. We recommend you to use capacitors with an allowable voltage which is at least twice as
much as setting output voltage. This is because there may be a case where a spike-shaped high voltage is
generated by an inductor when an external transistor is on and off.
l
Choose an inductor that has sufficiently small D.C. resistance and large allowable current and is hard to reach
magnetic saturation. And if the value of inductance of an inductor is extremely small, the I
LX
may exceed the
absolute maximum rating at the maximum loading.
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Rev. 1.11
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RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
