Simple Current Source Varies the Switching Frequency of a Power Supply for a Digital Still Camera
In some applications it is more efficient to power the core directly from the battery. In these applications VL will vary with the input battery voltage. The oscillator frequency is constant while VL is constant (Figure 1). A changing switching frequency will make it difficult to choose external components for optimum performance and efficiency for a given application. This can be resolved by using an external discrete current source in place of the external timing resistor.
Figure 1. MAX1802 oscillator frequency versus voltage.
Figure 2 shows an application circuit to power a PDA using the MAX1802 with a simple current source to set the switching frequency. The current source consists of two matched transistors P1 and P2 (Central Semiconductor's CMPT3906 transistors) and resistor R1.
Figure 2. The MAX1802 circuit using discrete current source to determine oscillator frequency.
The transistor N1 (Central Semiconductor's CMPT3904) is used to ensure the current source does not draw any current when the MAX1802 is shutdown. Note that the 3.8V output is not regulated above 4V inputs but that was acceptable in this particular application since this was used in conjunction with a linear post regulator (not shown) in the final system. This circuit operates with a switching frequency of 340kHz. Figure 3 gives the efficiency for the complete power supply system. The input voltage is varies from 2.6V to 5.5V to cover the use of Li+ battery or an adapter as input to the MAX1802. As can be seen from Figure 3 this circuit using the MAX1802 is an efficient and compact solution for this application.
Figure 3. Efficiency curve for circuit of Figure 1.