650 kHz /1.3 MHz Step-Up PWM DC-to-DC Switching Converters / ADP1612 | Theory of Operation
THEORY OF OPERATION
BG BAND GAP
Figure 34. Block Diagram with Step-Up Regulator Application Circuit
The ADP1612/ADP1613 current-mode step-up switching converters boost a 1.8 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 high 650 kHz/1.3 MHz switching frequency allows for the use of tiny external components.
The switch current is monitored on a pulse-by-pulse basis to limit it to 1.4 A typical (ADP1612) or 2.0 A typical (ADP1613).
CURRENT-MODE PWM OPERATION
The ADP1612/ADP1613 utilize 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 to the internal 1.235 V reference by the internal transconductance error amplifier to create an error voltage at COMP. The switch current is internally measured and added to the stabilizing ramp. The resulting sum is compared to 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.
The frequency of the ADP1612/ADP1613 is pin-selectable to operate at either 650 kHz to optimize the regulator for high efficiency or at 1.3 MHz for use with small external components.
If FREQ is left floating, the part defaults to 650 kHz. Connect
FREQ to GND for 650 kHz operation or connect FREQ to VIN for 1.3 MHz operation. When connected to VIN for 1.3 MHz operation, an additional 5 μA, typical, of quiescent current is active. This current is turned off when the part is shutdown.
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. Once the ADP1612/ADP1613 are turned on, SS sources
5 µA, typical, to the soft start capacitor (C
) until it reaches
1.2 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 ADP1612/
ADP1613 are in shutdown mode (EN ≤ 0.3 V), a thermal shutdown event occurs, or the input voltage is below the falling undervoltage lockout voltage, SS is internally shorted to GND to discharge the soft start capacitor.
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THERMAL SHUTDOWN (TSD)
The ADP1612/ADP1613 include 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 reduces 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 ADP1612/
ADP1613 automatically turn off the power switch and place 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 input turns the ADP1612/ADP1613 regulator on or off. Drive EN low to turn off the regulator and reduce the input current to 0.01 µA, typical. Drive EN high to turn on the regulator.
When the step-up dc-to-dc switching 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
circuit modification to disconnect the output voltage from the input voltage at shutdown.
Regardless of the state of the EN pin, when a voltage is applied to
VIN of the ADP1612/ADP1613, a large current spike occurs due to the nonisolated path through the inductor and diode between
. 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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