650 kHz/1.3 MHz, 4 A, Step-Up, PWM, DC-to-DC Switching Converter ADP1614 Data Sheet | THEORY OF OPERATION
THEORY OF OPERATION
The ADP1614 current-mode, step-up switching converter boosts a 2.5 V to 5.5 V input voltage to an output voltage as high as 20 V. The internal switch allows a high output current, and the 650 kHz/1.3 MHz switching frequency allows the use of
tiny external components. The switch current is monitored on a pulse-by-pulse basis to limit the current to the value set by the
resistor on the CLRES pin on the adjustable current-limit version or to 3 A typical on the fixed current-limit version.
1. THE PORTIONS IN THE DASHED BOXES DISPLAY THE TWO POSSIBLE FUNCTIONALITIES OF PIN 9 ON THE ADP1614.
Figure 32. Block Diagram with Step-Up Regulator Application Circuit
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CURRENT-MODE PWM OPERATION
The ADP1614 utilizes a current-mode PWM control scheme to regulate the output voltage over all load conditions. The output voltage is monitored at FB through a resistive voltage divider. The voltage at FB is compared with the internal 1.245 V reference by the internal transconductance error amplifier to create an error voltage at COMP. The current of the switch is internally measured and added to the stabilizing ramp. The resulting sum is compared with the error voltage at COMP to control the PWM modulator.
This current-mode regulation system allows fast transient response while maintaining a stable output voltage. By selecting the proper resistor-capacitor network from COMP to GND, the regulator response is optimized for a wide range of input voltages, output voltages, and load conditions.
ADJUSTABLE CURRENT LIMIT
A key feature of the ADP1614ACPZ-650-R7 and
ADP1614ACPZ-1.3-R7 is a pin-adjustable peak current limit of
current limit allows the other external components to be selected specifically for the application. The current limit is set via an external resistor connected from Pin 9 (CLRES) to ground. For the ADP1614ACPZ-R7 , the current limit is fixed at 3 A.
60 75 90 105
Figure 33. Peak Current Limit of Switch vs. R
The adjustable current-limit versions of the ADP1614 are internally programmed to operate at either 650 kHz or 1.3 MHz.
Operation of the ADP1614 at 650 kHz ( ADP1614ACPZ-650-R7 ) optimizes the efficiency of the device, whereas operation of the
ADP1614 at 1.3 MHz ( ADP1614ACPZ-1.3-R7 ) enables the device to be used with smaller external components. For the fixed current-limit version ( ADP1614ACP-R7 ), the frequency is pin selectable via the FREQ Pin (Pin 9). Connect FREQ to
GND for 650 kHz operation or connect FREQ to VIN for
1.3 MHz operation. Do not leave the FREQ pin floating.
To prevent input inrush current to the converter when the part is enabled, connect a capacitor from SS to GND to set the soft start period. After the ADP1614 is turned on, SS sources 5 µA
(typical) to the soft start capacitor (C
) until it reaches 1.23 V at startup. As the soft start capacitor charges, it limits the peak current allowed by the part. By slowly charging the soft start capacitor, the input current ramps slowly to prevent it from overshooting excessively at startup. When the ADP1614 is disabled, the SS pin is internally shorted to GND to discharge the soft start capacitor.
THERMAL SHUTDOWN (TSD)
The ADP1614 includes TSD protection. If the die temperature exceeds 150°C (typical), TSD turns off the NMOS power device, significantly reducing power dissipation in the device and preventing output voltage regulation. The NMOS power device remains off until the die temperature is reduced to 130°C (typical).
The soft start capacitor is discharged during TSD to ensure low output voltage overshoot and inrush currents when regulation resumes.
UNDERVOLTAGE LOCKOUT (UVLO)
If the input voltage is below the UVLO threshold, the ADP1614 automatically turns off the power switch and places the part into a low power consumption mode. This prevents potentially erratic operation at low input voltages and prevents the power device from turning on when the control circuitry cannot operate it. The UVLO levels have ~100 mV of hysteresis to ensure glitch-free startup.
The EN pin turns the ADP1614 regulator on or off. Drive EN low to shut down the regulator and reduce the input current to
0.25 µA (typical). Drive EN high to turn on the regulator.
When the converter is in shutdown mode (EN ≤ 0.3 V), there is a dc path from the input to the output through the inductor and output rectifier. This causes the output voltage to remain slightly below the input voltage by the forward voltage of the rectifier, preventing the output voltage from dropping to ground when the regulator is shut down.
Regardless of the state of the EN pin, when a voltage is applied to the VIN pin, a large current spike occurs due to the nonisolated path through the inductor and diode between V
. The high current is a result of the output capacitor charging. The peak value is dependent on the inductor, output capacitor, and any load active on the output of the regulator.
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