CAT3200HU2_14 Datasheet, PDF (7/9 Pages) ON Semiconductor – Low Noise Regulated Charge Pump DC-DC Converter

CAT3200HU2
Pin Functions
VIN is the power supply. During normal operation the
device draws a supply current which is almost constant. A
very brief interval of non−conduction will occur at the
switching frequency. The duration of the non−conduction
interval is set by the internal non−overlapping
“break−before−make” timing. VIN should be bypassed with
a 1 mF to 4.7 mF low ESR (Equivalent Series Resistance)
ceramic capacitor.
For filtering, a low ESR ceramic bypass capacitor (1 mF)
in close proximity to the IN pin prevents noise from being
injected back into the power supply.
SHDN is the logic control input (active low) that places the
device into shutdown mode. The internal logic is CMOS and
the pin does not use an internal pull−down resistor. The
SHDN pin should not be allowed to float.
CPOS, CNEG pins are the positive and negative
connections respectively for the charge pump flying
capacitor. A low ESR ceramic capacitor (1 mF) should be
connected between these pins. During initial power−up it
may be possible for the capacitor to experience a voltage
reversal and for this reason, avoid using a polarized
(tantalum or aluminum) flying capacitor.
VOUT is the regulated output voltage to power the load.
During normal operation, the device will deliver a train of
current pulses to the pin at a frequency of 2 MHz. Adequate
filtering on the pin can typically be achieved through the use
of a low ESR ceramic bypass capacitor (1 mF to 4.7 mF) in
close proximity to the VOUT pin. The ESR of the output
capacitor will directly influence the output ripple voltage.
When the shutdown mode is entered, the output is
immediately isolated from the input supply, however, the
output will remain connected to the internal feedback
resistor network (400 kW). The feedback network will result
in a reverse current of 10 mA to 20 mA to flow back through
the device to ground.
Whenever the device is taken out of shutdown mode, the
output voltage will experience a slew rate controlled
power−up. Full operating voltage is typically achieved in
less than 0.5 ms.
SGND is the ground reference for all voltages on the
CAT3200HU2.
FB is the feedback input pin. An output divider should be
connected from VOUT to FB to program the output voltage
when used in adjustable output mode. When used in 5 V
fixed output mode, connect the FB pin directly to GND.
PGND is the power ground.
Device Operation
The CAT3200HU2 uses a switched capacitor charge
pump to boost the voltage at IN to a regulated output voltage.
Regulation is achieved by sensing the output voltage
through an internal resistor divider (FB pin = GND) and
modulating the charge pump output current based on the
error signal. A 2−phase non−overlapping clock activates the
charge pump switches. The flying capacitor is charged from
the IN voltage on the first phase of the clock. On the second
phase of the clock it is stacked in series with the input voltage
and connected to VOUT. The charging and discharging the
flying capacitor continues at a free running frequency of
typically 2 MHz.
In shutdown mode all circuitry is turned off and the
CAT3200HU2 draws only leakage current from the VIN
supply. VOUT is disconnected from VIN. The SHDN pin is
a CMOS input with a threshold voltage of approximately
0.8 V. The CAT3200HU2 is in shutdown when a logic LOW
is applied to the SHDN pin. The SHDN pin is a high
impedance CMOS input. SHDN does not have an internal
pull−down resistor and should not be allowed to float. It
must always be driven with a valid logic level.
Short−Circuit and Thermal Protection
The CAT3200HU2 has built−in short−circuit current
limiting and over temperature protection. During overload
conditions, output current is limited to approximately
225 mA. At higher temperatures, or if the input voltage is
high enough to cause excessive chip self heating, the thermal
shutdown circuit shuts down the charge pump as the junction
temperature exceeds approximately 160°C. Once the
junction temperature drops back to approximately 140°C,
the charge pump is enabled. The CAT3200HU2 will cycle in
and out of thermal shutdown indefinitely without latch−up
or damage until a short−circuit on VOUT is removed.
Programming the CAT3200HU2 Output Voltage (FB Pin)
The CAT3200HU2 version has an internal resistive
divider to program the output voltage. The programmable
CAT3200HU2 may be set to an arbitrary voltage via an
external resistive divider. Since it employs a voltage
doubling charge pump, it is not possible to achieve output
voltages greater than twice the available input voltage.
Figure 20 shows the required voltage divider connection.
The voltage divider ratio is given by the formula:
R1
R2
+
V_OUT
1.27 V
*
1
Typical values for total voltage divider resistance can
range from several kW up to 1 MW.
1
3
CPOS CNEG
2
IN
8
OUT
6
SHDN
7
FB
4
PGND
5
SGND
ǒ Ǔ V_OUT + 1.27 V
1
)
R1
R2
R1
C_OUT
R2
Figure 20. Programming the Adjustable Output
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