DN507 - High Voltage Inverting Charge Pump Produces Low Noise Positive and Negative Supplies

High Voltage Inverting Charge Pump Produces Low Noise
Positive and Negative Supplies – Design Note 507
Marty Merchant
Inverting Charge Pump
The LTC3260 can supply up to 100mA from the inverted
input voltage at its charge pump output, VOUT. VOUT
also serves as the input supply to a negative LDO
regulator, LDO –. The charge pump frequency can be
adjusted between 50kHz to 500kHz by a single external
resistor. The MODE pin is used to select between a high
efficiency Burst Mode operation or constant frequency
mode to satisfy low noise requirements.
12V
4.7µF
1µF
LDO+
VIN
LTC3260
EN+
ADJ+
EN–
BYP+
MODE
GND
+
–
C
BYP
C–
ADJ–
4.7µF
10nF
10nF
316k
5V
ILDO+ ≤ 50mA
100k
100k
316k
–12V
10µF
LDO–
VOUT
RT
4.7µF
dn507 F01
|IVOUT| ≤ 100mA – |ILDO–|
200k
Figure 1. Typical 12V to ±5V Supply
10/12/507
–5V
|ILDO–| ≤ 50mA
Constant Frequency Mode
A single resistor at the RT pin sets the constant operating
frequency of the charge pump. If the RT pin is grounded,
the charge pump operates at 500kHz, where the openloop output resistance (ROL) and the output ripple are
optimized, allowing maximum available output power
with only a few millivolts peak-to-peak output ripple.
Light load efficiency can be increased by reducing the
operating frequency, as shown in Figure 2, but at the
expense of increased output ripple. The lower operating
frequency produces a higher effective open-loop resistance (ROL), but the reduced switching rate also reduces
the input current, resulting in increased efficiency at
light loads. Furthermore, at relatively heavy loads, the
increased ROL reduces the effective difference between
VOUT and LDO – —decreasing the power dissipation in
the negative LDO. The cumulative result is higher overall
efficiency with high input voltages and/or light loads.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
EFFICIENCY (%)
Introduction
Dual-polarity supplies are commonly needed to operate electronics such as op amps, drivers, or sensors,
but there is rarely a dual-polarity supply available at
the point of load. The LTC ®3260 is an inverting charge
pump (inductorless) DC/DC converter with dual low
noise LDO regulators that can produce positive and
negative supplies from a single wide input (4.5V to
32V) power source. It can switch between high efficiency Burst Mode ® operation and low noise constant
frequency mode, making it attractive for both portable
and noise-sensitive applications. The LTC3260 is available in a low profile 3mm × 4mm DFN or a thermally
enhanced 16-lead MSOP, yielding compact solutions
with minimal external components. Figure 1 shows a
typical 12V to ±5V application featuring the LTC3260.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
VOUT
LDO–
OSC = 50kHz
OSC = 200kHz
OSC = 500kHz
1
10
ILDO– (mA)
100
dn507 F02
Figure 2. LTC3260 VIN to VOUT and VIN to LDO–
Efficiency vs Frequency for the Circuit in Figure 1
Reducing the frequency increases the output ripple as
shown by the expression below and in Figure 3.
VRIPPLE(PK−PK) ≈
IOUT • tOFF
COUT
VOUT
100mV/DIV
 1

where tOFF = 
– 1µs
 fOSC

In general, constant frequency mode is suitable for
applications requiring low output ripple even at light
loads, but further gains in light load efficiency can be
gained by using Burst Mode operation, described below.
500kHz
200kHz
VOUT
20mV/DIV
50kHz
5µs/DIV
LTC3260 RIPPLE COMPARISON
VIN = 12V, IOUT = 20mA
dn507 F03
Figure 3. VOUT Constant Frequency Ripple Comparison
at 500kHz, 200kHz and 50kHz at 20mA Load
Burst Mode Operation
Figure 4 shows the light-load efficiency of the charge
pump in Burst Mode operation. Burst Mode operation
increases the output ripple over constant frequency
mode, but the increase in ripple is only a small percentage of VIN, as shown in Figure 5.
EFFICIENCY (%)
80
75
70
65
VIN = 12V
Burst Mode OPERATION
500kHz
LDO+ DISABLED
LDO– = –5V, 0A
10
IVOUT (mA)
1
100
dn507 F04
100µs/DIV
LTC3260 VOUT RIPPLE
VIN = 12V, IOUT = 10mA
Burst Mode OPERATION 500kHz
dn507 F05
Figure 5. VOUT Ripple in Burst Mode Operation
Burst Mode operation is implemented by charging VOUT
close to –VIN. The LTC3260 then enters a low quiescent
current sleep state, about 100µA with both LDO regulators enabled, until the burst hysteresis is reached. Then
the charge pump wakes up and the cycle repeats. The
average VOUT is approximately –0.94VIN. As the load
increases, the charge pump runs more often to keep
the output in regulation. If the load increases enough,
the charge pump automatically switches to constant
frequency mode in order to maintain regulation.
Dual LDOs
Both of the LTC3260’s LDOs — the positive LDO regulator supplied from VIN, and the negative LDO regulator
supplied from VOUT — are capable of supporting 50mA
loads. Each LDO has a dropout voltage of 300mV with
a 50mA output and has an adjust pin, allowing the
output voltage to be set by a simple resistor divider.
The LDO regulators can be individually enabled. The
EN – pin enables both the inverting charge pump and
LDO –. When both regulators are disabled, the part
shuts down with only 2µA of quiescent current. The
LDO references can be filtered by adding a capacitor
on each of the bypass pins to further reduce noise at
the LDO regulator outputs.
Conclusion
The LTC3260 produces low noise positive and negative supplies from a single positive power source. The
LTC3260 features optional Burst Mode operation for
light-load efficiency in battery-powered devices, or
low noise constant frequency mode for noise-sensitive
applications. The LTC3260’s combination of inverting
charge pump and dual LDO regulators yields elegant
solutions to applications with 4.5V to 32V inputs.
Figure 4. LTC3260 Burst Mode Operation Efficiency
Data Sheet Download
www.linear.com/3260
Linear Technology Corporation
For applications help,
call (978) 656-4700, Ext. 4778
dn507f LT/AP 1012 196K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2012
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