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S-8460B00AFT-TBG

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33
STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.2.1_00
S-8460
Seiko Instruments Inc. 7
Operation
1. Step-up or Step-down DC-DC Converter
1. 1 Basic Operation
The S-8460 automatically selects step-up operation or step-down operation to hold the output voltage constant
according to the input voltage (V
IN
), output voltage (V
OUT
), and output current (I
OUT
). A high-efficient power supply
can be constructed using the S-8460, since the S-8460 works as a switching regulator for both step-up and step-
down operation.
Figure 6 shows the block diagram of the S-8460.
Internal circuits operate on the internal voltage (V
L
) except pre-driver circuit for EXT1 and ON/OFF circuit. When
V
IN
is 4.5 V or more, the voltage is stepped down to 4.5 V and when V
IN
is lower than 4.5 V, V
L
is set to V
IN
. The
output voltage of the pre-driver circuit for EXT1 lies between the BST pin voltage V
BST
and the LX pin voltage V
LX
where the BST pin voltage V
BST
is normally V
LX
+ V
L
. The gate to source voltages for all external MOS FETs,
SW1 to SW3, thus become V
L
, which drives these external MOS FETs.
FB
EXT3
PWM step-up and
step-down selection
VSS DVSS CPRO CSS
VL
EXT2
LX
VIN BST
EXT1
+
−
ON / OFF
Voltage
source
Oscillation
circuit
Overload
protection
Reference voltage
with soft-start
Figure 6
STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8460
Rev.2.1_00
8
Seiko Instruments Inc.
1. 2 Step-up Operation
Parasitic diode
SW2
LX1 SW1
C
OUT
V
OUT
SD1
V
IN
SW3
ON/OFF
Figure 7
Step-up operation is carried out by setting SW1 : ON, SW2 : OFF, and toggling SW3. The voltage V
IN
+ V
L
is
needed at the BST pin to turn SW1 on. For this purpose, the BST pin voltage (C
BST
) is charged to the V
L
voltage
by the switch combination SW1 : OFF, SW2 : ON for approximately 200 ns just after SW3 is turned off, and the
BST pin is then bootstrapped to V
IN
+ V
L
by SW1 : ON, SW2 : OFF.
SW2 is turned on after SW1 is turned off and SW1 is turned on after SW2 is turned off to avoid the large current to
flow between V
IN
and V
SS
if SW1 and SW2 are turned on simultaneously. When the two switches, SW1 and SW2,
are turned off, current flows to V
OUT
through the parasitic diode of SW2. In some MOS FETs current is not
allowed to flow through the parasitic diode. In this case, a Schottky diode must be connected parallel to the MOS
FET.
1. 3 Step-down Operation
SW2
ON/OFF
SW1 ON/OFF
C
OUT
V
OUT
SD1
V
IN
SW3
OFF
Parasitic diode
Figure 8
Step-down operation is carried out by synchronous switching of SW1 and SW2, and keeping SW3 open.
The BST pin voltage is kept at V
IN
+ V
L
, since the switches, SW1 and SW2, repeat toggling in each period in step-
down operation.
STEP-UP OR DOWN, SINGLE-COIL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.2.1_00
S-8460
Seiko Instruments Inc. 9
1. 4 Control Sequence
SW2
SW1
C
OUT
V
OUT
SD1
V
IN
SW3
Parasitic diode
Figure 9
If the switches, SW1 and SW2, are turned on simultaneously, V
IN
and V
SS
are short-circuited and large useless
current flows. And if the switches, SW2 and SW3, are turned on simultaneously, the energy stored in the coil
flows to V
SS
and is wasted. The S-8460 thus controls the switches in such a way that in operations involving SW1
and SW2, and involving SW2 and SW3 both switches are turned off simultaneously to avoid useless current flowing
due to simultaneous turn-on of the switches.
1. 5 Step-up and Step-down Selection Control
The S-8460 automatically selects operation between step-up and step-down to maintain a constant output voltage
according to the relationship between the input voltage (V
IN
), output voltage (V
OUT
), and output current (I
OUT
).
Simple relationships that step-up operation work when input voltage ≤ output voltage and that step-down operation
work when input voltage ≥ output voltage do not hold. Step-up operation emerges when the output voltage is kept
constant by step-up operation, and step-down operation emerges when the output voltage is kept constant by step-
down operation according to the relationship between V
IN
, V
OUT
, and I
OUT
.
Figure 10 shows the turning point between step-up operation and step-down schematically for the case when the
output voltage is 3.3 V. In the area where the two slant lines are crossing and noted by “Step-up or down” the S-
8460 shows step-up operation or step-down operation. Not that step-up operation and step-down operation
appear alternately in this area, but that one of the two operations is selected and stable operation is carried out.
The voltage for the turning point between step-up and step-down varies slightly due to external parts and mounting
conditions.
Step-down operation
18
3.3
2.2
0.1 1.0 10.0 100.0 1000.0
I
OUT
(mA)
V
IN
(V)
Output voltage
Step-up or
down
Step-up operation
OUT
=
3.3 V
Figure 10 Graphic Scheme for Automatic Selection of Step-up and Step-down
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S-8460B00AFT-TBG

Mfr. #:
Buy S-8460B00AFT-TBG
Manufacturer:
ABLIC
Description:
Switching Controllers 2.5-6.0V Step-Up/Dwn
Lifecycle:
New from this manufacturer.
Delivery:
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