SLVS294F September 2000 – August 2015 TPS62000 , TPS62002 , TPS62003 , TPS62004 , TPS62005 , TPS62006 , TPS62007 , TPS62008
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Device Comparison Table
- 6 Pin Configuration and Functions
- 7 Specifications
- 8 Detailed Description
-
9 Application and Implementation
- 9.1 Application Information
- 9.2 Typical Application
- 9.3
System Examples
- 9.3.1 Standard 5-V to 3.3-V/600-mA Conversion; High Efficiency
- 9.3.2 Single Li-ion to 2.5-V/600-mA Using Ceramic Capacitors Only
- 9.3.3 Single Li-ion to 1.8 V/300 mA; Smallest Solution Size
- 9.3.4 Dual Cell NiMH or NiCd to 1.2 V/200 mA; Smallest Solution Size
- 9.3.5 Dynamic Output Voltage Programming As Used in Low Power DSP Applications
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information
8 Detailed Description
8.1 Overview
The TPS6200x is a step down converter operating in a current mode PFM/PWM scheme with a typical switching frequency of 750 kHz.
At moderate to heavy loads, the converter operates in the pulse width modulation (PWM) and at light loads the converter enters a power save mode (pulse frequency modulation, PFM) to keep the efficiency high.
In the PWM mode operation, the part operates at a fixed frequency of 750 kHz. At the beginning of each clock cycle, the high side P-channel MOSFET is turned on. The current in the inductor ramps up and is sensed via an internal circuit. The high side switch is turned off when the sensed current causes the PFM/PWM comparator to trip when the output voltage is in regulation or when the inductor current reaches the current limit (set by ILIM). After a minimum dead time preventing shoot through current, the low side N-channel MOSFET is turned on and the current ramps down again. As the clock cycle is completed, the low side switch is turned off and the next clock cycle starts.
In discontinuous conduction mode (DCM), the inductor current ramps to zero before the end of each clock cycle. In order to increase the efficiency the load comparator turns off the low side MOSFET before the inductor current becomes negative. This prevents reverse current flowing from the output capacitor through the inductor and low side MOSFET to ground that would cause additional losses.
As the load current decreases and the peak inductor current does not reach the power save mode threshold of typically 120 mA for more than 15 clock cycles, the converter enters a pulse frequency modulation (PFM) mode.
In the PFM mode, the converter operates with:
- Variable frequency
- Constant peak current that reduces switching losses
- Quiescent current at a minimum
Thus maintaining the highest efficiency at light load currents. In this mode, the output voltage is monitored with the error amplifier. As soon as the output voltage falls below the nominal value, the high side switch is turned on and the inductor current ramps up. When the inductor current reaches the peak current of typical: 150 mA + 50 mA/V × (VIN – VOUT), the high side switch turns off and the low side switch turns on. As the inductor current ramps down, the low side switch is turned off before the inductor current becomes negative which completes the cycle. When the output voltage falls below the nominal voltage again, the next cycle is started.
The converter enters the PWM mode again as soon as the output voltage can not be maintained with the typical peak inductor current in the PFM mode.
The control loop is internally compensated reducing the amount of external components.
The switch current is internally sensed and the maximum current limit can be set to typical 600 mA by connecting ILIM to ground; or, to typically 1.2 A by connecting ILIM to VIN.
8.2 Functional Block Diagram
8.3 Feature Description
8.3.1 Low Noise Antiringing Switch
An antiringing switch is implemented in order to reduce the EMI radiated from the converter during discontinuous conduction mode (DCM). In DCM, the inductor current ramps to zero before the end of each switching period. The internal load comparator turns off the low side switch at that instant thus preventing the current flowing backward through the inductance which increases the efficiency. An antiringing switch across the inductor prevents parasitic oscillation caused by the residual energy stored in the inductance (see Figure 11).
NOTE
The antiringing switch is only activated in the fixed output voltage versions. It is not enabled for the adjustable output voltage version TPS62000.
8.3.2 Enable
Logic low on EN forces the TPS6200x into shutdown. In shutdown, the power switch, drivers, voltage reference, oscillator, and all other functions are turned off. The supply current is reduced to less than 1 μA in the shutdown mode.
8.3.3 Undervoltage Lockout
An undervoltage lockout circuit provides the save operation of the device. It prevents the converter from turning on when the voltage on VIN is less than typically 1.6 V.
8.3.4 Power Good Comparator
The power good (PG) comparator has an open drain output capable of sinking typically 10 μA. The PG is only active when the device is enabled (EN = high). When the device is disabled (EN = low), the PG pin is high impedance.
The PG output is only valid after a 100 μs delay after the device is enabled and the supply voltage is greater than 1.2 V. This is only important in cases where the pullup resistor of the PG pin is connected to an external voltage source which might cause an initial spike (false high signal) within the first 100 μs after the input voltage exceeds 1.2 V. This initial spike can be filtered with a small R-C filter to avoid false power good signals during start-up.
If the PG pin is connected to the output of the TPS62000 with a pullup resistor, no initial spike (false high signal) occurs and no precautions have to be taken during start-up.
The PG pin becomes active high when the output voltage exceeds typically 94.5% of its nominal value. Leave the PG pin unconnected when not used.
8.4 Device Functional Modes
The TPS6200x is a synchronous current-mode PWM converter with integrated – and P-channel power MOSFET switches. Synchronous rectification is used to increase efficiency and to reduce external component count. To achieve the highest efficiency over a wide load current range, the converter enters a power-saving pulse-frequency modulation (PFM) mode at light load currents. Operating frequency is typically 750 kHz, allowing the use of small inductor and capacitor values. The device can be synchronized to an external clock signal in the range of 500 kHz to 1 MHz. For low-noise operation, the converter can be operated in the PWM mode and the internal antiringing switch reduces noise and EMI. In the shutdown mode, the current consumption is reduced to less than 1 μA. The TPS62000 is available in the 10-pin (DGS) microsmall outline package (VSSOP). The device operates over a free-air temperature range of –40°C to 85°C.
8.4.1 Soft Start
As the enable pin (EN) goes high, the soft-start function generates an internal voltage ramp. This causes the start-up current to slowly rise preventing output voltage overshoot and high inrush currents. The soft-start duration is typical 1 ms (see Figure 12). When the soft-start function is completed, the error amplifier is connected directly to the internal voltage reference.
8.4.2 Synchronization, Power Save Mode, and Forced PWM Mode
If no clock signal is applied, the converter operates with a typical switching frequency of 750 kHz. It is possible to synchronize the converter to an external clock within a frequency range from 500 kHz to 1000 kHz. The device automatically detects the rising edge of the first clock and is synchronized immediately to the external clock. If the clock signal is stopped, the converter automatically switches back to the internal clock and continues operation without interruption. The switch over is initiated if no rising edge on the SYNC pin is detected for a duration of four clock cycles. Therefore, the maximum delay time can be 8 μs in case the internal clock has a minimum frequency of 500 kHz.
In case the device is synchronized to an external clock, the power save mode is disabled and the device stays in forced PWM mode.
Connecting the SYNC pin to the GND pin enables the power save mode. The converter operates in the PWM mode at moderate to heavy loads and in the PFM mode during light loads maintaining high efficiency over a wide load current range.
Connecting the SYNC pin to the VIN pin forces the converter to operate permanently in the PWM mode even at light or no load currents. The advantage is the converter operates with a fixed switching frequency that allows simple filtering of the switching frequency for noise sensitive applications. In this mode, the efficiency is lower compared to the power save mode during light loads (see ).
It is possible to switch from forced PWM mode to the power save mode during operation.
The flexible configuration of the SYNC pin during operation of the device allows efficient power management by adjusting the operation of the TPS6200x to the specific system requirements.
8.4.3 100% Duty Cycle Operation
As the input voltage approaches the output voltage and the duty cycle exceeds typical 95%, the converter turns the P-channel high side switch continuously on. In this mode, the output voltage is equal to the input voltage minus the voltage drop across the P-channel MOSFET.
8.4.4 No Load Operation
In case the converter operates in the forced PWM mode and there is no load connected to the output, the converter will regulate the output voltage by allowing the inductor current to reverse for a short period of time.