TPS62135 Datasheet, PDF (10/40 Pages) Texas Instruments – High Accuracy 3-V to 17-V 3.5-A Step-Down Converters with DCS-Control

TPS62135, TPS621351
SLVSBH3A – JUNE 2016 – REVISED SEPTEMBER 2016
www.ti.com
9.4 Device Functional Modes
9.4.1 Pulse Width Modulation (PWM) Operation
TPS62135x has two operating modes: Forced PWM mode discussed in this section and PWM/PFM as discussed
in Power Save Mode Operation (PWM/PFM).
With the MODE pin set to high, the TPS62135x operates with pulse width modulation in continuous conduction
mode (CCM) with a nominal switching frequency of 2.5 MHz. The frequency variation in PWM is controlled and
depends on VIN, VOUT and the inductance. The on-time in forced PWM mode is given by:
TON = ^VOUT ´ 400[ns]
VIN
(1)
9.4.2 Power Save Mode Operation (PWM/PFM)
When the MODE pin is low, Power Save Mode is allowed. The device operates in PWM mode as long the output
current is higher than half the inductor's ripple current. To maintain high efficiency at light loads, the device
enters Power Save Mode at the boundary to discontinuous conduction mode (DCM). This happens if the output
current becomes smaller than half the inductor's ripple current. For improved transient response, PWM mode is
forced for 8 switching cycles if the output voltage is above target due to a load release. The Power Save Mode is
entered seamlessly, if the load current decreases and the MODE pin is set low. This ensures a high efficiency in
light load operation. The device remains in Power Save Mode as long as the inductor current is discontinuous.
In Power Save Mode the switching frequency decreases linearly with the load current maintaining high efficiency.
The transition into and out of Power Save Mode is seamless in both directions.
The AEE function in TPS62135 and TPS621351 adjust the on-time (TON) in power save mode depending on the
input voltage and the output voltage to maintain highest efficiency. The on-time, in steady-state operation, can be
estimated as:
TON = 100´ ^VIN [ns]
VIN -VOUT
(2)
For very small output voltages, an absolute minimum on-time of about 50 ns is kept to limit switching losses. The
operating frequency is thereby reduced from its nominal value, which keeps efficiency high. Using TON, the
typical peak inductor current in Power Save Mode is approximated by:
IL P S M ( peak ) = ^{(V IN - V O U T )} ´ T O N
L
(3)
There is a minimum off-time which limits the duty cycle of the TPS62135x. When V_IN decreases to typically 15%
above VOUT, the TPS62135x does not enter Power Save Mode, regardless of the load current. The device
maintains output regulation in PWM mode.
The output voltage ripple in power save mode is given by Equation 4:
DV
=
L ´VIN ²
200´ C
çæ
è ^VIN
1
- VOUT
+¹
VOUT
÷ö
ø
(4)
9.4.3 100% Duty-Cycle Operation
The duty cycle of the buck converter is given by D = VOUT/VIN and increases as the input voltage comes close
to the output voltage. The minimum off-time is about 80 ns. When the minimum off-time is reached, TPS62135x
scales down its switching frequency while it approaches 100% mode. In 100% mode it keeps the high-side
switch on continuously. The high-side switch stays turned on as long as the output voltage is below the internal
set point. This allows the conversion of small input to output voltage differences, for example for longest
operation time of battery-powered applications. In 100% duty cycle mode, the low-side FET is switched off.
The minimum input voltage to maintain output voltage regulation, depending on the load current and the output
voltage level, can be calculated as:
spacing
( ) VIN (min) = VOUT + IOUT RDS(on) + RL
(5)
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