UC3844BS

UC3844 is a current-mode PWM IC designed for switching power supplies. Compared to voltage-mode control, it offers superior load transient response and line regulation. The device is available in SOP8 and DIP8 packages. Its error amplifier provides 0.5mA push-pull output current. Peak current is defined as I_S(MAX) ≈ 1.0V / R_s; a small RC filter network is recommended to suppress switching transients. The oscillator timing capacitor CT is charged through RT from VREF and discharged by an internal current source. During discharge, the internal clock signal drives the output low. Both the oscillator period and maximum duty cycle can be set simultaneously by selecting RT and CT. Charge and discharge times are determined by the following equations: t_c ≈ 0.55 RT × CT; t_d = RT × CT × ln((0.063 RT - 2.7)/(0.063 RT - 4)). The resulting frequency is: f = (t_c + t_d)^(-1). When RT > 5kΩ, f ≈ 1.8/(RT × CT). The device provides two shutdown methods: raising pin 3 voltage above 1V, or pulling pin 1 voltage low to near ground (approximately two diode forward voltage drops). Both methods force the PWM comparator output high and take priority in resetting the PWM latch, holding the output low until the next clock cycle after the shutdown signal is removed. Slope compensation can be implemented in converters requiring duty cycles above 50% by summing a portion of the oscillator ramp into the current sense signal. A filter comprising capacitor C and resistor R2 is recommended to suppress leading-edge switching spikes.

Applications

• Inverter power supplies
• High-power high-frequency switching power supplies
• DC pulse-width modulation speed control systems
• UPS power supplies
• DC/DC converters

Key features

• Built-in undervoltage lockout circuit
• Low startup current
• Stable internal reference voltage source
• High-current push-pull output
• Operating frequency up to 500kHz
• Automatic feed-forward compensation
• Double pulse suppression
• Enhanced load response characteristics