Datasheet
THEORY OF OPERATION
Peak Current Reduction
OFF SWITCHED
Step-Down(Buck)Mode
OFF ON
C
PUMP
Q
1
Q
3
Q
2
Q
4
C
IN
C
OUT
V
OUT
V
IN
Step-Up(Boost)Mode
(A) (B)
OFF
ON
ON
OFF
C
PUMP
Q
2
Q
4
C
OUT
V
OUT
C
PUMP
Q
2
Q
4
C
OUT
V
OUT
OFF
ON
Q
1
Q
3
ON
OFF
Q
1
Q
3
C
IN
V
IN
C
IN
V
IN
REG71055-Q1
SBAS477 – JULY 2009 .......................................................................................................................................................................................................
www.ti.com
The REG71055 regulated charge pump provides a regulated output voltage for input voltages ranging from less
than the output to greater than the output. This is accomplished by automatic mode switching within the device.
When the input voltage is greater than the required output, the unit functions as a variable frequency
switch-mode regulator. This operation is shown in Figure 8 . Transistors Q
1
and Q
3
are held off, Q
4
is on, and Q
2
is switched as needed to maintain a regulated output voltage.
When the input voltage is less than the required output voltage, the device switches to a step-up or boost mode
of operation, as shown in Figure 9 .
A conversion clock of 50% duty cycle is generated. During the first half cycle the FET switches are configured as
shown in Figure 9 A, and C
PUMP
charges to V
IN
.
During the second half cycle the FET switched are configured as shown in Figure 9 B, and the voltage on C
PUMP
is added to V
IN
. The output voltage is regulated by skipping clock cycles as necessary.
In normal operation, the charging of the pump and output capacitors usually leads to relatively high peak input
currents which can be much higher than that of the average load current. The regulator incorporates circuitry to
limit the input peak current, lowering the total EMI production of the device and lowering output voltage ripple and
input current ripple. Input capacitor (C
IN
) supplies most of the charge required by input current peaks.
Figure 8. Simplified Schematic of the REG71055 Operating in the Step-Down Mode
Figure 9. Simplified Schematic of the REG71055 Operating in the Step-Up or Boost Mode
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Product Folder Link(s): REG71055-Q1