Diodes AP3591 - Manual (Page 11)

AP3591
Document number: DS36906 Rev. 1 - 2
11 of 18
www.diodes.com
March 2014
© Diodes Incorporated
AP3591
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A Product Line of
Diodes Incorporated
Application Information
1. Functional Description
The AP3591 is a synchronous step-down controller. Adaptive constant on time (COT) control is employed to provide fast transition response and
easy loop stabilization. AP3591 does not have a dedicated in board oscillator. It runs with a pseudo-constant frequency which is set by R
ON
. The
output voltage variation is sensed by FB Pin. If V
FB
is below 0.75V, the error comparator will trigger the control logic and generate an ON-time
period, in which high side MOSFET is turned on and low side MOSFET is turned off. The ON-time period length is calculated using the following
equation:
f


IN
OUT
ON
V
V
t
V
OUT
is the output voltage, V
IN
is the input voltage, and f is the switching frequency.
The on-time is the time required for the voltage on this capacitor charging from zero volts to V
OUT
, thereby the ON-time of the high side switch is
directly proportional to the output voltage and inversely proportional to the input voltage. The implementation results in a nearly constant switching
frequency without the need of a clock generator.
t
ON
= 14.5p×R
TON
×(V
OUT
+0.1)/V
IN
+50ns
After an ON-time period, the AP3591 goes into the OFF-time period. The OFF-time period length depends on V FB in most case. It will end when
V
FB
decreases below 0.75V and then the ON-time period is triggered again. If the OFF-time period is less than the minimum OFF time, the
minimum OFF time will be applied, which is about 400ns typical.
2. Mode Selection Operation
AP3591 has two operation modes: Continuous Conduction Mode (CCM) and Diode Emulation Mode (DEM). When the EN/DEM pin voltage is
higher than 2.9V, AP3591 will operate in DEM mode for high efficiency; when the EN/DEM pin is floating, AP3591 will operate in forced CCM
mode to a certain frequency during a light load condition.
2.1 Diode Emulation Mode
If the DEM mode is selected, the AP3591 automatically reduces the switching frequency under a light load condition to get high efficiency. When
the output current decreases and heavy load condition is formed, the inductor current decreases as well, and eventually comes close to zero
current, which is the boundary between CCM and DEM. The low side MOSFET will turn off whenever the inductor current reaches zero level. The
load is provided only by the output capacitor. When FB voltage is lower than 0.75V, the next ON cycle is beginning. The ON-time is kept the same
as that in the heavy load condition. The switching frequency increases to keep V
OUT
voltage when the output current increases from light to heavy
load. The transition load point is calculated using the following equation:
ON
t



2L
V V
I OUT IN
LOAD
t ON is the on-time.
2.2 Continuous Conduction Mode
When AP3591 operates in CCM mode, the duty cycle V
OUT
/V
IN
is not changed at light load condition. The low side MOSFET keeps on even when
inductor current decreases to reverse. The benefit of CCM is to keep the switching frequency fairly constant to avoid a certain frequency during a
light load condition.
3. Power On Reset and Soft-start
Power on reset occurs when V
DD
rises above approximately 3.9V: the IC will reset the fault latch and prepare the PWM for operation. When V
DD
is
below 3.6V, the VDD under voltage lockout (UVLO) circuitry inhibits switching by keeping UGATE and LGATE low. A built-in soft-start is used to
prevent surge current from power supply input V IN during turn on (referring to Functional Block Diagram). The error amplifier is a three-input device.
Reference voltage V
REF
or the internal soft-start voltage V
SS
whichever is smaller dominates the behavior of the non-inverting inputs of the error
amplifier. V
SS
internally ramps up to 95% of 0.75V in 1.2ms for AP3591 after the soft-start cycle is initiated.
Figure 1 shows a typical start-up interval for AP3591 when the EN/DEM pin has been released from a grounded (system shutdown) state.