Datasheet

(nF) x V (nF) x 0.765 1.1
REF
SS SS
t (ms) = =
SS
I ( A)
SS
C 1.1 C
6
´
m
´
( )
IN OUT OUT
OUT(LL)
SW IN
V -V ×V
1
I = ×
2×L×f V
TPS54526
www.ti.com SLVSB84B –MAY 2012–REVISED JANUARY 2014
At the beginning of each cycle, the high-side MOSFET is turned on. The MOSFET is turned off after the internal
one-shot timer expires. The one-shot timer is set by the converter input voltage, VIN, and the output voltage, VO,
to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control.
The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below
the reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the
need for ESR induced output ripple from D-CAP2™ mode control.
PWM Frequency and Adaptive On-Time Control
TPS54526 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The
TPS54526 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to
set the on-time one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the
output voltage, therefore, when the duty ratio is VOUT/VIN, the frequency is constant.
Auto-Skip Eco-Mode™ Control
The TPS54526 is designed with Auto-Skip Eco-mode™ to increase light load efficiency. As the output current
decreases from heavy load condition, the inductor current is also reduced and eventually comes to point that its
rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous
conduction modes. The rectifying MOSFET is turned off when its zero inductor current is detected. As the load
current further decreases the converter run into discontinuous conduction mode. The on-time is kept almost the
same as it was in the continuous conduction mode so that it takes longer time to discharge the output capacitor
with smaller load current to the level of the reference voltage. The transition point to the light load operation
I
OUT(LL)
current can be calculated in Equation 1.
(1)
Soft Start and Pre-Biased Soft Start
The soft start function is adjustable. When the EN pin becomes high, 6 μA current begins charging the capacitor
which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start up.
The equation for the slow start time is shown in Equation 2. VFB voltage is 0.765 V and SS pin source current is
6 μA.
(2)
The TPS54526 contains a unique circuit to prevent current from being pulled from the output during startup if the
output is pre-biased. When the soft-start commands a voltage higher than the pre-bias level (internal soft start
becomes greater than feedback voltage V
FB
), the controller slowly activates synchronous rectification by starting
the first low side FET gate driver pulses with a narrow on-time. It then increments that on-time on a cycle-by-
cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle of the converter. This
scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (VO) starts and ramps
up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to normal
mode operation.
Power Good
The TPS54526 has power-good open drain output. The power good function is activated after soft start has
finished. The power good function becomes active after 1.7 times soft-start time. When the output voltage is
within -10% of the target value, internal comparators detect power good state and the power good signal
becomes high. Rpg resister value ,which is connected between PG and VREG5, is required from 25kΩ to 150kΩ.
If the feedback voltage goes under 15% of the target value, the power good signal becomes low after a 5 μs
internal delay.
Copyright © 2012–2014, Texas Instruments Incorporated 7
Product Folder Links :TPS54526