CM4072 Datasheet, PDF (9/11 Pages) California Micro Devices Corp – Low Noise Charge Pump/Linear Regulator LED Driver

PRELIMINARY
CM4072
Application Information
Ripple Frequency
The charge pump internal oscillation frequency is
about 250kHz. However, this is the continuous, free-
running frequency, which is usually only seen while the
charge pump is powering up. Such a sawtooth 'ripple'
waveform on CS can have a much lower frequency
than 250kHz. This mode of operation is necessary to
conserve power. If it were not done this way, then a
much larger package with heatsink would be required.
The frequency of this 'ripple' is affected by V_IN, I_OUT,
C_S capacitor value and C_P capacitor value.
Guidelines for External Capacitors
(1) To find C_P: specify value of V_IN, and highest value
of I_OUT:
If V_IN= 3.3V +/- 5%, then minimum value
of C_P (µF) = I_OUT (mA) / 85.
If V_IN= 5.0V +/- 10%, then minimum value
of C_P (µF) = I_OUT (mA) / 700
(2) The V_IN decoupling capacitor, C_IN, should typically
be much greater than C_P to prevent voltage droop dur-
ing C_P charging. Excessive glitches on V_IN will affect
the output voltage V_OUT. C_IN is typically 10X greater
than C_P.
(3) C_S should be small to ensure that the ripple fre-
quency is high, but at least 2x greater than C_P, other-
wise the ripple amplitude will be very high. Reducing
the value of C_S will increase the ripple frequency.
Examples of C_S ripple frequencies (C_S=10μF,
T_A=25°C) are shown in following tables:
C_P = 0.47μF
V_IN
I_OUT C_S Frequency
3.14
15mA
46kHz
3.60
15mA
35kHz
4.50
70mA
76kHz
5.50
70mA
56kHz
C_P = 1.0μF
V_IN
I_OUT C_S Frequency
3.14
100mA
250kHz
3.60
100mA
110kHz
4.50
100mA
67kHz
5.50
100mA
49kHz
(4) C_OUT, the optional V_OUT capacitor, helps minimize
noise and improve load regulation; 0.1µF to 100µF is
recommended.
(5) C_BYP, the optional bypass capacitor helps reduce
noise in the LDO; 0.1µF is recommended.
After choosing external component values, check in-
system performance (at min/max V_IN, max tempera-
ture, and min/max I_OUT). See the troubleshooting guide
on next page for tips if there are problems.
Charge Pump Noise
The charge pump is 'digital' in operation and can pro-
duce digital noise at both the free-running frequency
and at the ripple frequency.
To minimize noise, PCB grounding is important! This
part requires short, low-impedance ground connec-
tions for DGND (pin 1), GND (pin 4), the V_IN decou-
pling capacitor (pin 2), the C_S capacitor (pin 3), the
Bypass decoupling capacitor (pin 5) and the V_OUT
decoupling capacitor (pin 8). All decoupling capacitors
and the C_S capacitor should be low-ESR ceramics.
The C_P capacitor needs to be low-ESR.
© 2005 California Micro Devices Corp. All rights reserved.
11/08/05
490 N. McCarthy Blvd., Milpitas, CA 95035-5112 ● Tel: [phone] ● Fax: [phone] ● www.cmd.com 9