PRELIMINARY
CALIFORNIA MICRO DEVICES
Applications Information
CM3702
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. After the charge pump output voltage (CS) reaches
approximately 5.8V, the charge pump pauses until the CS voltage drops to approximately 5.7V. Then the charge pump
restarts and runs until the CS voltage is greater than approximately 5.8V, when it pauses again, and this process repeats.
This gives rise to a sawtooth ‘ripple’ waveform on CS which 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, Cs capacitor value and Cp capacitor value.
Guidelines for choosing values for external capacitors.
(1) To find Cp: specify value of V_IN, and highest value of I_OUT:
If V_IN= 3.3V +/- 5%, then
minimum
value of Cp(µF) = I_OUT(mA) / 85
If V_IN= 5.0V +/- 10%, then
minimum
value of Cp(µF) = I_OUT(mA) / 700
(2) Ci, the V_IN decoupling capacitor, should typically be much greater than Cp to prevent voltage droop during Cp
charging.
Excessive glitches on V_IN will affect the output voltage V_OUT.
Typically Ci is 10X greater than Cp. But usually there are already some capacitors on this supply, so adding extra
capacitors is not necessary – simply move an already-present low-ESR capacitor close to the CM3702.
This is especially important for V_IN = 5V.
(3) Choose value of Cs. Cs should be small to ensure that the ripple frequency is high, but Cs should be at least 2x
greater than Cp otherwise the ripple amplitude will be very high. Reducing the value of Cs will increase the ripple
frequency.
Examples of Cs ripple frequencies: (Cs=10µF, 25°C)
Cp=0.47µF
Cp=1µF
V_IN=3.14, I_OUT=15mA CS Frequency=46kHz
V_IN=3.14, I_OUT=100mA
CS Frequency=250kHz
V_IN=3.60, I_OUT=15mA CS Frequency=35kHz
V_IN=3.60, I_OUT=100mA
CS Frequency=110kHz
V_IN=4.50, I_OUT=70mA CS Frequency=76kHz
V_IN=4.50, I_OUT=100mA
CS Frequency=67kHz
V_IN=5.50, I_OUT=70mA CS Frequency=56kHz
V_IN=5.50, I_OUT=100mA
CS Frequency=49kHz
(4) Co, the V_OUT decoupling capacitor helps minimize noise and improve load regulation. 0.1µF - 100µF
recommended.
(5) Cbyp, the bypass capacitor helps reduce noise in the LDO. 0.1µF recommended.
After choosing external component values, check in-system performance (at min/max V_IN, max temperature, and
min/max I_OUT). See troubleshooting guide on next page for tips if there are problems.
Charge Pump Noise
The charge pump is ‘digital’ in operation and can produce 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 connections for DGND
(pin 1), GND (pin 4), the V_IN decoupling capacitor (pin 2), the CS capacitor (pin 3), the Bypass decoupling capacitor (pin
5) and the V_OUT decoupling capacitor (pin 8). All decoupling capacitors and the Cs capacitor should be low-ESR
ceramics.
The Cp capacitor does NOT need to be low-ESR.
Efficiency
The power efficiency in % of the combined charge pump and LDO is approximately:
100 * (V
OUT
) / (V
IN
*2)
Power Dissipation
The dissipation of the part is approximately:
((V
IN
*2) – V
OUT
) * I
OUT
© 2004 California Micro Devices Corp. All rights reserved
09/22/04
430 N. McCarthy Blvd, #100, Milpitas, California 95035
Tel: (408) 263-3214 Fax: (408) 263-7846
www.calmicro.com
8
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
