CALIFORNIA MICRO DEVICES

PRELIMINARY

CM3702

© 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

Applications 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. 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:

Power Dissipation

The dissipation of the part is approximately: