PRELIMINARY
CALIFORNIA MICRO DEVICES
CM3702
The MSOP-10 package heats at a rate of about 200°C/W (θ
JA
). (Note that this value is approximate because it depends
upon the copper tracks and ground planes on the pcb.) If V_IN = 5V and I_OUT = 100mA then the power dissipation will
be approximately 500mW. Multiplying this by the
θ
JA
of 200, the part’s internal temperature will be about 100°C higher
than the ambient temperature. If the ambient temperature is 70°C then the internal temperature will be approximately
170°C which will typically trigger the overtemperature circuit and depower the part.
Internal temperature = Ambient temperature + (
θ
JA
* Power dissipation )
(Must be less than 170°C)
Note that the evaluation pcb has a
θ
JA
of less than 150°C/W, based upon measured performance.
How to reduce the power dissipation of the part, and how to get more than 100mA
If V_IN = 5V typ., then the charge pump / LDO combination is capable of providing more than 100mA. The only problem
is power dissipation.
If the input voltage is lowered using an external diode then the output current can be increased without causing the part to
overheat:
Using this circuit I_OUT can be 200mA if V_IN = 4.75V, and yet the part will not overheat even if V_IN = 5.25V,
I_OUT=200mA and the ambient temperature is 85°C.
Warnings
The charge pump output CS (pin 3) must not be shorted to GND or held below its internally-set voltage while the part is
powered. This usually results in the destruction of the part.
With V_IN = 5V, the maximum current that can be continuously drawn from CS is approximately 100mA dc.
Never short Cp+ (pin 9) to Cp- (pin 10). This will cause large currents to flow from V_IN to DGND through the part,
usually causing its destruction. This will happen even if EN_Chip and EN_LDO are off.
Troubleshooting Guide
1.
Is the output voltage is drooping under heavy loads? Perhaps the charge pump cannot provide the necessary current.
Try increasing the value of Cp. If that does not work then is V_IN too low? Is V_IN dropping during the Cp charging cycle?
If V_IN is not suitably decoupled and drops below 3.1V then the available current will be very low.
2.
Is the output voltage oscillating between 5V and 0V? The part may be reaching its overtemperature limit. Reduce
current consumption, reduce
θ
JA
or add an external diode on the input to reduce V_IN.
3.
Is the part too noisy? Try increasing value (or reducing ESR) of Cs, Ci, Co, Cb. At minimum current the charge pump
ripple frequency will be low. If V
OUT
noise is at the charge pump ripple frequency then change values of Cp and Cs.
Reducing the input voltage V
IN
will reduce the charge pump ripple frequency noise on V
OUT.
4.
Will the part power up? Pin 6 must be HIGH to power up. Even if pin 7 is HIGH, pin 6 must
also
be high to power up.
5.
Can the cold start power-up time be reduced? Yes, by reducing the value of the BYP capacitor.
© 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
9
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
