1

Denso

TPS54160-Q1 “low dropout” operation

Todd ToporskiAnalog Field Applications2/4/2011

2

TPS54160-Q1 Overview

• Buck circuit recommended to Denso• Description of “low dropout” operation• What “low dropout” condition looks like• Work-around used by Denso-Japan• TI recommendations

3

TPS54160-Q1 Circuit For DensoSwitcher Pro design:Vin_min = 7V, Vin_max = 24V (continuous)Vout = 5V, Iout_max = 500mA, Fsw = 400kHz

In “light load” conditions, with nearly 0A in the inductor, PH = VOUT (approximately)and D1 does NOT conduct during OFF cycle. Therefore, PH is not pulled below Vout to help charge BOOT cap during OFF cycle.

This is issue when VIN is low (within 2.1V of Vout).

4

Description of “low dropout” operationPage 14 of TPS54160-Q1 datasheet:

5

Description of “low dropout” operation(continued)

Page 14 of TPS54160-Q1 datasheet:

6

Description of “low dropout” operation (continued)Page 14 of TPS54160-Q1 datasheet:

7

Description of “low dropout” operation (continued)• To reduce power consumption in light load conditions, TPS54160-Q1 uses a BOOTSTRAP circuit for driving the gate of the internal highside FET.

• Another method to supply gate voltage for driving internal highside FET is to use a CHARGE PUMP.

• Disadvantage of CHARGE PUMP is it requires clock and consumes more quiescent current. Advantage of BOOTSTRAP is it does not require a clock, and it consumes less quiescent current.

• Advantage of CHARGE PUMP is it provides better FET drive in “low dropout” conditions. Disadvantage of BOOTSTRAP is it can be problematic in “low dropout” conditions for driving the FET.

• The BOOTSTRAP & “low dropout” limitation experienced on TPS54160-Q1 is similarly experienced on other buck switchers using BOOTSTRAP.

8

What “low dropout” condition looks like

Vin = 6VVout = 5VIout = 500uA

9

What “low dropout” condition looks likeVin = 6VVout = 5VIout = 500uA

In light load condition, Vin – Vout > 2.1V(or BOOT-PH > 2.1V) prevents switcherfrom reaching UVLO and turning off.

• For Vout = 5V, Vin < 7.1V, UVLO will be reached as BOOT cap discharges andswitcher turns off. Therefore, at Vin = 6V (this example) UVLO is reached.

• VOUT decays until Vin – Vout (or BOOT-PH) > 2.1V, then switcherturns on again to regulate to Vout = 5V.

As soon as boot cap discharges to< 2.1V, UVLO is reached again.

• Cycle repeats itself.

10

Work-around used by Denso – Japan• At Denso & TI meeting in Southfied, MI on Jan. 21, 2011, it was discussed with TI thatDenso-Japan had used a “work-around” for TPS54160 dropout issue.

• Below is example of the implementation using a diode and resistor (39k) on PH node.

* Denso mentioned that TI engineer in Japan came up with work-around circuit.

11

How “work-around” circuit works

During “normal” buck operation with current in inductor:Switch turns ON• Extra diode conducts and energizes inductor with current, D1 blocks current.• 39k resistor to GND sinks small current equal to VIN/39k.Switch turns OFF• D1 conducts & PH2 is diode voltage drop below GND• Extra diode conducts to bring PH1 close to PH2 voltage, bootstrap cap charges

During “light load current” condition with ~0A in inductorSwitch turns ON• Extra diode conducts and energizes inductor with current, D1 blocks current.• Output regulates to VOUT.Switch turns OFF• PH2 = VOUT since D1 does not conduct.• Extra diode blocks PH2 voltage so that 39k can pull PH1 towards GND to recharge BOOT cap so BOOT-PH > 2.1V

12

What “work-around” looks like

Vin = 6VVout = 5VIout = 500uA

• Vout regulates properly

• BOOT – PH reaches 3.76V during switch off cycle • Pulse-skipping regulation is maintained until load increases.

13

TI Recommendations & Summary

• Work-around circuit appears to work for helping dropout condition, but should be tested over all conditions deemed necessary by Denso.

• Drawback of “work-around” circuit is it dissipates more losses (across diode) when load current increases. For 500mA application these losses are not very large.

• For larger load currents, such as 1A or higher, the “work-around” circuit will dissipate more power across the diode and this should be accounted for in the design.