Datasheet

SYSTEM
CONTROLLER
LM3208
V
IN
V
OUT
0.8V to 3.6V
L1
3.3 PH
C1*
10 PF
* Place C1 close to PV
IN
C2
4.7 PF
V
DD
PV
IN
SW
FB
EN
V
CON
PGNDSGND
DAC
ON/OFF
2.7V to 5.5V
LM3208
www.ti.com SNVS404B –APRIL 2006–REVISED MARCH 2013
Figure 35. Typical Operating System Circuit
Circuit Operation
Referring to Figure 1 and Figure 34, the LM3208 operates as follows. During the first part of each switching
cycle, the control block in the LM3208 turns on the internal PFET (P-channel MOSFET) switch. This allows
current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the
current to a ramp with a slope of around (V
IN
– V
OUT
) / L, by storing energy in a magnetic field. During the second
part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns
the NFET (N-channel MOSFET) synchronous rectifier on. In response, the inductor’s magnetic field collapses,
generating a voltage that forces current from ground through the synchronous rectifier to the output filter
capacitor and load. As the stored energy is transferred back into the circuit and depleted, the inductor current
ramps down with a slope around V
OUT
/ L. The output filter capacitor stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across the load.
The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the
load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and
synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output
voltage is equal to the average voltage at the SW pin.
While in operation, the output voltage is regulated by switching at a constant frequency and then modulating the
energy per cycle to control power to the load. Energy per cycle is set by modulating the PFET switch on-time
pulse width to control the peak inductor current. This is done by comparing the signal from the current-sense
amplifier with a slope compensated error signal from the voltage-feedback error amplifier. At the beginning of
each cycle, the clock turns on the PFET switch, causing the inductor current to ramp up. When the current sense
signal ramps past the error amplifier signal, the PWM comparator turns off the PFET switch and turns on the
NFET synchronous rectifier, ending the first part of the cycle. If an increase in load pulls the output down, the
error amplifier output increases, which allows the inductor current to ramp higher before the comparator turns off
the PFET. This increases the average current sent to the output and adjusts for the increase in the load.
Before appearing at the PWM comparator, a slope compensation ramp from the oscillator is subtracted from the
error signal for stability of the current feedback loop. The minimum on time of PFET is 55ns (typ.)
Shutdown Mode
Setting the EN digital pin low (<0.5V) places the LM3208 in shutdown mode (0.01µA typ.). During shutdown, the
PFET switch, NFET synchronous rectifier, reference voltage source, control and bias circuitry of the LM3208 are
turned off. Setting EN high (>1.2V) enables normal operation.
EN should be set low to turn off the LM3208 during power-up and under voltage conditions when the power
supply is less than the 2.7V minimum operating voltage. The LM3208 is designed for compact portable
applications, such as mobile phones. In such applications, the system controller determines power supply
sequencing and requirements for small package size outweigh the additional size required for inclusion of UVLO
(Under Voltage Lock-Out) circuitry.
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