ISL9120IR
10
FN8743.1
February 2, 2016
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Automatic Bypass Mode Operation
When the output voltage is close to the input voltage, generally
within 1% to 2%, the ISL9120IR will engage automatic bypass
mode operation, which produces a direct connection between
the VIN and VOUT pins. This behavior provides excellent efficiency
and very low output voltage ripple.
Forced Bypass Mode Operation
Forced bypass mode operation is intended for applications where
the output regulation is not important but the device quiescent
current consumption is important. One example is when the
buck-boost regulator is providing power to a LDO and the LDO is
in standby mode with near zero output current. Under this
condition, putting the buck-boost regulator in the bypass mode
will have essentially no impact on the LDO but save the 41µA
quiescent current consumption on the buck-boost regulator.
Since the bypass mode is an extreme power saving mode, there
is no overcurrent protection. Therefore, caution must be taken
not to overload or short-circuit the device. Power-up in the bypass
mode is not recommended.
Output Voltage Programming
The ISL9120IR is available in fixed and adjustable output voltage
versions. To use the fixed output version (ISL9120IRNZ), the
VOUT pin must be connected directly to the FB pin.
In the adjustable output voltage version (ISL9120IRAZ), an
external resistor divider is required to program the output
voltage.
Applications Information
Component Selection
The fixed-output version (ISL9120IRNZ) requires only three
external power components to implement the buck-boost
converter: an inductor, an input capacitor and an output
capacitor.
The adjustable version (ISL9120IRAZ) requires three additional
components to program the output voltage. Two external
resistors program the output voltage and a small capacitor is
added to improve transient response.
Output Voltage Programming, Adjustable
Version
Setting and controlling the output voltage of the ISL9120IRAZ
(adjustable output version) can be accomplished by selecting the
external resistor values.
Equation 1
can be used to derive the R
1
and R
2
resistor values:
When designing a PCB, include a GND guard band around the FB
resistor network to reduce noise and improve accuracy and
stability. Resistors R
1
and R
2
should be positioned close to the
FB pin. The suggested value of the R
1
resistor is 187k.
Feed-Forward Capacitor Selection
A small capacitor (C
4
in Figure 17) in parallel with resistor R
1
is
required to provide the specified load and line regulation. The
suggested value of this capacitor is 22pF for R
1
=187k. An NPO
type capacitor is recommended.
Non-Adjustable Version FB Pin Connection
The fixed output version of the ISL9120IR does not require
external resistors or a capacitor on the FB pin. Simply connect
VOUT to FB, as shown in Figure 18
.
V
OUT
= 3.3V
VOUT
FB
C
2
47µF
R
1
R
2
187kΩ
60.4kΩ
VIN
V
IN
=
1.8V TO 5.5V
EN
BYPS
C
1
10µF
ISL9120IRAZ
GND
PGND
LX1
LX2
L
1
1µH
C
4
22pF
FORCED BYPASS
BUCK-BOOST
ENABLE
DISABLE
FIGURE 17. TYPICAL ISL9120IRAZ APPLICATION
V
OUT
0.8V 1
R
1
R
2
-------
+
=
(EQ. 1)
FIGURE 18. TYPICAL ISL9120IRNZAPPLICATION
V
OUT
= 3.3V
VOUT
FB
C
2
22µF or 47µF
VIN
V
IN
=
1.8V TO 5.5V
C
1
10µF
ISL9120IRNZ
GND
PGND
LX1
LX2
L
1
1µH
EN
BYPS
BYPASS
BUCK-BOOST
ENABLE
DISABLE
