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PDF CAT7117 Data sheet ( Hoja de datos )
| Número de pieza | CAT7117 | |
| Descripción | Dual Channel 800mA High Efficiency Step-Down DC/DC Converter | |
| Fabricantes | Chip Advanced Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de CAT7117 (archivo pdf) en la parte inferior de esta página. Total 12 Páginas | ||
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CAT7117
Dual Channel 800mA High Efficiency Step-Down DC/DC Converter
Confidentiality Statement:
This Data Sheet (including any attachments) is for the sole use of the intended
recipient(s) and contains confidential, proprietary and privileged information. Any
unauthorized review, use, disclosure or distribution is prohibited. If you are not the
intended recipient, please immediately destroy all copies of the original message.
1 page  
Dual Channel 800mA High Efficiency Step-Down DC/DC Converter
CAT7117
Function Description
The CAT7117 is a constant frequency current mode
PWM step-down converter. The CAT7117 is
optimized for low voltage, Li-Ion battery powered
applications where high efficiency and small size
are critical. The CAT7117 uses an external resistor
divider to set the output voltage from 0.6V to 5.5V.
The device integrates both a main switch and a
synchronous rectifier, which provides high efficiency
and eliminates an external Schottky diode. The
CAT7117 can achieve 100% duty cycle. The duty
cycle D of a step-down converter is defined as:
D = TON * FOSC * 100% ≒ VOUT/VIN * 100%
where TON is the main switch on time, and FOSC
is the oscillator frequency.
Current Mode PWM Control
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle
current limit for superior load and line response and
protection of the internal main switch and
synchronous rectifier. The CAT7117 switches at a
constant frequency (1.8MHz) and regulates the
output voltage. During each cycle the PWM
comparator modulates the power transferred to the
load by changing the inductor peak current based
on the feedback error voltage. During normal
operation, the main switch is turned on for a certain
time to ramp the inductor current at each rising
edge of the internal oscillator, and switched off
when the peak inductor current is above the error
voltage. When the main switch is off, the
synchronous rectifier will be turned on immediately
and stay on until either the next cycle starts or the
inductor current drops to zero. The device skips
pulses to improve efficiency at light load.
Internal soft-start
The CAT7117 features an internal soft−start
function, which reduces inrush current and
overshoot of the output voltage. Soft−start is
achieved by ramping up the reference voltage (Vref)
which is applied to the input of the error amplifier.
The typical soft−start time is about 2 msec and
depends on components values on AP circuit.
Dropout Operation
The CAT7117 allows the main switch to remain on
for more than one switching cycle and increases the
duty cycle while the input voltage is dropping close
to the output voltage. When the duty cycle reaches
100%, the main switch is held on continuously to
deliver current to the output up to the P MOSFET
current limit. The output voltage then is the input
voltage minus the voltage drop across the main
switch and the inductor.
Short Circuit Protection
The CAT7117 has short circuit protection. When the
output is shorted to ground, the oscillator frequency
is reduced to prevent the inductor current from
increasing beyond the P MOSFET current limit. The
P MOSFET current limit is also reduced to lower the
short circuit current. The frequency and current limit
will return to the normal values once the short
circuit condition is removed and the feedback
voltage reaches 0.6V.
Maximum Load current
The CAT7117 can operate down to 2.5V input
voltage; however the maximum load current
decreases at lower input due to large IR drop on the
main switch and synchronous rectifier. The slope
compensation signal reduces the peak inductor
current as a function of the duty cycle to prevent
sub-harmonic oscillations at duty cycles greater
than 50%. Conversely the current limit increases as
the duty cycle decreases match other diagrams in
this datasheet.
Input Capacitor Selection
The input capacitor has to sustain the ripple current
produced during the on time of the upper MOSFET,
so it must have a low ESR to minimize the losses.
The RMS value of this ripple is:
I IN RMS = IOUT D × (1 − D)
where D is the duty cycle, linRMS is the input RMS
current, and IOUT is the load current. The equation
reaches its maximum value with D = 0.5. Losses in
the input capacitors can be calculated with the
following equation:
PCIN = ESR CIN × I IN RMS 2
The CAT logo is a registered trademark of Chip Advanced Technology
2007 Chip Advanced Technology Inc. – All Right Reserved.
01/03/2008 Rev:1.5 5/12
5 Page 
Dual Channel 800mA High Efficiency Step-Down DC/DC Converter
CAT7117
The CAT logo is a registered trademark of Chip Advanced Technology
2007 Chip Advanced Technology Inc. – All Right Reserved.
01/03/2008 Rev:1.5 11/12
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet CAT7117.PDF ] | |
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