CM4072 Datasheet, PDF (10/11 Pages) California Micro Devices Corp – Low Noise Charge Pump/Linear Regulator LED Driver

PRELIMINARY
CM4072
Efficiency
The power efficiency in % of the combined charge
pump and LDO is approximately:
100 x (V_OUT) / (V_IN x 2)
Power Dissipation
The dissipation of the part is approximately:
((V_IN x 2) - V_OUT) x I_OUT
The TDFN-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 gives an internal temperature about
100°C higher than the ambient temperature (T_A). If the
T_A is 70°C then the internal temperature will be
approximately 170°C which will trigger the overtemper-
ature circuit and lead to power-down.
Internal temperature = Ambient temperature
+ ( θ_JA x 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 combina-
tion 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 caus-
ing the part to overheat. The circuit below illustrates an
example of how to increase the output current.
1
10
1N4006
5V
±10%
+
C_i
10μF
+ C_S
2
9
3
CM4072
TDFN-10
8
C_BYP +
3μF
4
7
0.1μF
5
6
+ 1Cμ^PF
+ _CO ^VOUT
0.1μF
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 V_CP (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 con-
tinuously drawn from V_CP is approximately 100mA dc.
Never short C_P+ (pin 9) to C_P- (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_CP and EN_LDO are off.
Troubleshooting Guide
1) Is the output voltage drooping under heavy loads?
Perhaps the charge pump cannot provide the neces-
sary current. Try increasing the value of C_P. If that
does not work, then, is V_IN too low? Is V_IN dropping
during the C_P 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 the value (or
reducing ESR) of C_S, C_IN, C_S, C_B. 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 C_P and C_S. Reducing the input volt-
age 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 C_BYP.
Enable
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