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| 型号： | TPS59124RGET |
| 厂商： | TEXAS INSTRUMENTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, PQCC24 |
| 封装： | GREEN, PLASTIC, VQFN-24 |
| 文件页数： | 23/25页 |
| 文件大小： | 846K |
| 代理商： | TPS59124RGET |
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f
IN
OUT
OUT LL
IN
V
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I
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L
V
TPS59124
www.ti.com
SLUSA58 – JULY 2010
DETAILED DESCRIPTION
PWM OPERATION
The main control loop of the switching mode power supply (SMPS) is designed as an adaptive on-time pulse
width modulation (PWM) controller. It supports a proprietary D-CAP Mode. D-CAP Mode uses an internal
compensation circuit and is suitable for low external component-count configuration, with appropriate amount of
ESR at the output capacitor(s). The output voltage is monitored at a feedback point voltage. The reference
voltage at the feedback point is a combination of a fixed 0.750-V precision reference and a synchronized,
precision 15-mV ramp signal. Lower output voltages in notebook systems (e.g., 1.05 V, 1.5 V) require extremely
low output ripple. By providing a ramp signal, the TPS59124 is easier to use in low-output ripple systems. The
combination of the precision ramp and reference yield an effective target reference of 0.758 V. The accuracy of
this effective reference remains 1.3% over line and temperature.
At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This
MOSFET is turned off, or becomes OFF state, after the internal one-shot timer expires. This one shot is
determined by the converter’s input voltage, VIN, and the output voltage, VOUT, to keep the frequency fairly
constant over the input voltage range; hence, it is called adaptive on-time control (see PWM Frequency and
Adaptive On-time Control). The high-side MOSFET is turned on again when feedback information indicates
insufficient output voltage, and inductor current information indicates a below-the-over-current limit condition.
Repeating operation in this manner, the controller regulates the output voltage. The synchronous low-side
MOSFET is turned on each OFF state to keep the conduction loss at a minimum. The low-side MOSFET is
turned off when the inductor current information detects zero level. This enables seamless transition to the
reduced frequency operation at light-load conditions so that high efficiency is kept over a broad range of load
current.
LIGHT-LOAD CONDITION
TPS59124 automatically reduces switching frequency at light-load conditions to maintain high efficiency. This
reduction of frequency is achieved smoothly and without increase of Vout ripple or load regulation. Detail
operation is described as follows. As the output current decreases from heavy-load condition, the inductor
current is also reduced, and eventually comes to the point that its valley touches zero current, which is the
boundary between continuous conduction and discontinuous conduction modes. The low-side MOSFET is turned
off when this zero inductor current is detected. As the load current is further decreased, the converter runs in
discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that
requires the next ON cycle. The ON time is kept the same as that in the heavy-load condition. In reverse, when
the output current increases from light load to heavy load, the switching frequency increases to the preset value
as the inductor current reaches the continuous conduction. The transition load point to the light-load operation,
IOUT(LL) (i.e., the threshold between continuous and discontinuous conduction mode) can be calculated in
where
f is the PWM switching frequency
(1)
Switching frequency versus output current in the light-load condition is a function of L, f, VIN, and VOUT, but it
It should be noted that in the PWM control path is a small ramp . This ramp is transparent in normal, continuous
conduction mode and does not measurably affect the regulation voltage. However, in discontinuous, light-load
mode, an upward shift in regulation voltage of about 0.75% will be observed. The variation of this shift minimally
affects the reference tolerance. Therefore, the reference value in skip mode is 0.764 V ±1.3% over line and
temperature.
Copyright 2010, Texas Instruments Incorporated
7
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